configure, meson: move C++ compiler detection to meson.build

The test is slightly weaker than before, because it does not
call an extern "C" function from a C source file.  However,
in practice what we seek to detect is ABI compatibility of the
various sanitizer flags, and for that it is enough to compile
anything with CC and link it with CXX.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -75,7 +75,6 @@ fi
 TMPB="qemu-conf"
 TMPC="${TMPDIR1}/${TMPB}.c"
 TMPO="${TMPDIR1}/${TMPB}.o"
-TMPCXX="${TMPDIR1}/${TMPB}.cxx"
 TMPM="${TMPDIR1}/${TMPB}.m"
 TMPE="${TMPDIR1}/${TMPB}.exe"
 
@@ -158,10 +157,6 @@ do_cc() {
     do_compiler_werror "$cc" $CPU_CFLAGS "$@"
 }
 
-do_cxx() {
-    do_compiler_werror "$cxx" $CPU_CFLAGS "$@"
-}
-
 do_objc() {
     do_compiler_werror "$objcc" $CPU_CFLAGS "$@"
 }
@@ -171,24 +166,6 @@ add_to() {
     eval $1=\${$1:+\"\$$1 \"}\$2
 }
 
-update_cxxflags() {
-    # Set QEMU_CXXFLAGS from QEMU_CFLAGS by filtering out those
-    # options which some versions of GCC's C++ compiler complain about
-    # because they only make sense for C programs.
-    QEMU_CXXFLAGS="-D__STDC_LIMIT_MACROS -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS"
-    CONFIGURE_CXXFLAGS=$(echo "$CONFIGURE_CFLAGS" | sed s/-std=gnu11/-std=gnu++11/)
-    for arg in $QEMU_CFLAGS; do
-        case $arg in
-            -Wstrict-prototypes|-Wmissing-prototypes|-Wnested-externs|\
-            -Wold-style-declaration|-Wold-style-definition|-Wredundant-decls)
-                ;;
-            *)
-                QEMU_CXXFLAGS=${QEMU_CXXFLAGS:+$QEMU_CXXFLAGS }$arg
-                ;;
-        esac
-    done
-}
-
 compile_object() {
   local_cflags="$1"
   do_cc $CFLAGS $EXTRA_CFLAGS $CONFIGURE_CFLAGS $QEMU_CFLAGS $local_cflags -c -o $TMPO $TMPC
@@ -2362,38 +2339,6 @@ fi
 #######################################
 # generate config-host.mak
 
-# Check that the C++ compiler exists and works with the C compiler.
-# All the QEMU_CXXFLAGS are based on QEMU_CFLAGS. Keep this at the end to don't miss any other that could be added.
-if has $cxx; then
-    cat > $TMPC <<EOF
-int c_function(void);
-int main(void) { return c_function(); }
-EOF
-
-    compile_object
-
-    cat > $TMPCXX <<EOF
-extern "C" {
-   int c_function(void);
-}
-int c_function(void) { return 42; }
-EOF
-
-    update_cxxflags
-
-    if do_cxx $CXXFLAGS $EXTRA_CXXFLAGS $CONFIGURE_CXXFLAGS $QEMU_CXXFLAGS -o $TMPE $TMPCXX $TMPO $QEMU_LDFLAGS; then
-        # C++ compiler $cxx works ok with C compiler $cc
-        :
-    else
-        echo "C++ compiler $cxx does not work with C compiler $cc"
-        echo "Disabling C++ specific optional code"
-        cxx=
-    fi
-else
-    echo "No C++ compiler available; disabling C++ specific optional code"
-    cxx=
-fi
-
 if ! (GIT="$git" "$source_path/scripts/git-submodule.sh" "$git_submodules_action" "$git_submodules"); then
     exit 1
 fi
@@ -2474,7 +2419,6 @@ echo "MESON=$meson" >> $config_host_mak
 echo "NINJA=$ninja" >> $config_host_mak
 echo "CC=$cc" >> $config_host_mak
 echo "QEMU_CFLAGS=$QEMU_CFLAGS" >> $config_host_mak
-echo "QEMU_CXXFLAGS=$QEMU_CXXFLAGS" >> $config_host_mak
 echo "QEMU_OBJCFLAGS=$QEMU_OBJCFLAGS" >> $config_host_mak
 echo "GLIB_CFLAGS=$glib_cflags" >> $config_host_mak
 echo "GLIB_LIBS=$glib_libs" >> $config_host_mak
@@ -2665,7 +2609,6 @@ if test "$skip_meson" = no; then
       echo "${a}-softmmu = '$c'" >> $cross
   done
 
-  test -z "$cxx" && echo "link_language = 'c'" >> $cross
   echo "[built-in options]" >> $cross
   echo "c_args = [$(meson_quote $CFLAGS $EXTRA_CFLAGS)]" >> $cross
   echo "cpp_args = [$(meson_quote $CXXFLAGS $EXTRA_CXXFLAGS)]" >> $cross

meson.build

@@ -180,7 +180,6 @@ endif
 ##################
 
 qemu_cflags = config_host['QEMU_CFLAGS'].split()
-qemu_cxxflags = config_host['QEMU_CXXFLAGS'].split()
 qemu_objcflags = config_host['QEMU_OBJCFLAGS'].split()
 qemu_ldflags = config_host['QEMU_LDFLAGS'].split()
 
@@ -194,7 +193,6 @@ endif
 
 if get_option('gprof')
   qemu_cflags += ['-p']
-  qemu_cxxflags += ['-p']
   qemu_objcflags += ['-p']
   qemu_ldflags += ['-p']
 endif
@@ -240,8 +238,33 @@ if get_option('fuzzing')
 endif
 
 add_global_arguments(qemu_cflags, native: false, language: ['c'])
-add_global_arguments(qemu_cxxflags, native: false, language: ['cpp'])
 add_global_arguments(qemu_objcflags, native: false, language: ['objc'])
+
+# Check that the C++ compiler exists and works with the C compiler.
+link_language = 'c'
+linker = cc
+qemu_cxxflags = []
+if add_languages('cpp', required: false, native: false)
+  cxx = meson.get_compiler('cpp')
+  add_global_arguments(['-D__STDC_LIMIT_MACROS', '-D__STDC_CONSTANT_MACROS', '-D__STDC_FORMAT_MACROS'],
+                       native: false, language: 'cpp')
+  foreach k: qemu_cflags
+    if k not in ['-Wstrict-prototypes', '-Wmissing-prototypes', '-Wnested-externs',
+                 '-Wold-style-declaration', '-Wold-style-definition', '-Wredundant-decls']
+      qemu_cxxflags += [k]
+    endif
+  endforeach
+  add_global_arguments(qemu_cxxflags, native: false, language: 'cpp')
+
+  if cxx.links(files('scripts/main.c'), args: qemu_cflags)
+    link_language = 'cpp'
+    linker = cxx
+  else
+    message('C++ compiler does not work with C compiler')
+    message('Disabling C++-specific optional code')
+  endif
+endif
+
 add_global_link_arguments(qemu_ldflags, native: false, language: ['c', 'cpp', 'objc'])
 
 if targetos == 'linux'
@@ -255,14 +278,6 @@ add_project_arguments('-iquote', '.',
                       '-iquote', meson.current_source_dir() / 'include',
                       language: ['c', 'cpp', 'objc'])
 
-link_language = meson.get_external_property('link_language', 'cpp')
-if link_language == 'cpp'
-  add_languages('cpp', required: true, native: false)
-  cxx = meson.get_compiler('cpp')
-  linker = cxx
-else
-  linker = cc
-endif
 if host_machine.system() == 'darwin'
   add_languages('objc', required: false, native: false)
 endif

scripts/main.c

@@ -0,0 +1 @@
+int main(void) {}