gdb: Pass -x c++ to the compiler

Because we are compiling .c files containing C++ code, clang++ complains
with:

  clang: error: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated

If renaming all the source files to .cpp is out of the question, an
alternative is to pass "-x c++" to convince the compiler that we are
really compiling C++.  It works fine with GCC too.

gdb/ChangeLog:

	* Makefile.in (COMPILE.pre): Add "-x c++".

gdb/gdbserver/ChangeLog:

	* Makefile.in (COMPILE.pre): Add "-x c++".

gdb/ChangeLog

@@ -1,3 +1,7 @@
+2017-06-17  Simon Marchi  <simon.marchi@ericsson.com>
+
+	* Makefile.in (COMPILE.pre): Add "-x c++".
+
 2017-06-16  Alan Hayward  <alan.hayward@arm.com>
 	    Pedro Alves  <palves@redhat.com>
 	    Yao Qi  <yao.qi@linaro.org>

gdb/Makefile.in

@@ -114,7 +114,7 @@ depcomp = $(SHELL) $(srcdir)/../depcomp
 
 # Note that these are overridden by GNU make-specific code below if
 # GNU make is used.  The overrides implement dependency tracking.
-COMPILE.pre = $(CXX) $(CXX_DIALECT)
+COMPILE.pre = $(CXX) -x c++ $(CXX_DIALECT)
 COMPILE.post = -c -o $@
 COMPILE = $(COMPILE.pre) $(INTERNAL_CFLAGS) $(COMPILE.post)
 POSTCOMPILE = @true

gdb/gdbserver/ChangeLog

@@ -1,3 +1,7 @@
+2017-06-17  Simon Marchi  <simon.marchi@ericsson.com>
+
+	* Makefile.in (COMPILE.pre): Add "-x c++".
+
 2017-06-09  Sergio Durigan Junior  <sergiodj@redhat.com>
 
 	* fork-child.c: Conditionally include <signal.h>.

gdb/gdbserver/Makefile.in

@@ -65,7 +65,7 @@ depcomp = $(SHELL) $(srcdir)/../depcomp
 
 # Note that these are overridden by GNU make-specific code below if
 # GNU make is used.  The overrides implement dependency tracking.
-COMPILE.pre = $(CXX) $(CXX_DIALECT)
+COMPILE.pre = $(CXX) -x c++ $(CXX_DIALECT)
 COMPILE.post = -c -o $@
 COMPILE = $(COMPILE.pre) $(INTERNAL_CFLAGS) $(COMPILE.post)
 POSTCOMPILE = @true