libstdc++: Only add valid -L paths to testsuite linker options

The MacOS linker warns about -L arguments that don't exist, which causes
all tests to fail for the defauly configuration (because libbacktrace
isn't built).

libstdc++-v3/ChangeLog:

	* scripts/testsuite_flags.in: Only add src/filesystem/.libs and
	src/libbacktrace/.libs to LDFLAGS if those directories exist.

libstdc++-v3/scripts/testsuite_flags.in

@@ -77,9 +77,15 @@ case ${query} in
       echo ${PCHFLAGS}
       ;;
     --cxxldflags)
-      SECTIONLDFLAGS="@SECTION_LDFLAGS@ @LIBICONV@
-                      -L${BUILD_DIR}/src/filesystem/.libs
-                      -L${BUILD_DIR}/src/libbacktrace/.libs"
+      FS_LDFLAGS=
+      BT_LDFLAGS=
+      if [ -d ${BUILD_DIR}/src/filesystem/.libs ]; then
+        FS_LDFLAGS=-L${BUILD_DIR}/src/filesystem/.libs
+      fi
+      if [ -d ${BUILD_DIR}/src/libbacktrace/.libs ]; then
+        BT_LDFLAGS=-L${BUILD_DIR}/src/libbacktrace/.libs
+      fi
+      SECTIONLDFLAGS="@SECTION_LDFLAGS@ @LIBICONV@ $FS_LDFLAGS $BT_LDFLAGS"
       echo ${SECTIONLDFLAGS}
       ;;
     *)