gdb: Respect CXXFLAGS when building with C++ compiler

Currently, even when built with --enable-build-with-cxx, gdb uses
CFLAGS instead of CXXFLAGS.  This commit fixes it.

CXXFLAGS set in the environment when configure was run is now honored
in the generated gdb/Makefile, and you can also override CXXFLAGS in
the command like at make time, with the usual 'make CXXFLAGS="..."'

Objects built with a C compiler (e.g., gnulib) still honor CFLAGS
instead.

gdb/ChangeLog:
2016-01-21  Pedro Alves  <palves@redhat.com>

	* Makefile.in (COMPILER_CFLAGS): New.
	(CXXFLAGS): Get it from configure.
	(INTERNAL_CFLAGS_BASE, INTERNAL_LDFLAGS): Use COMPILER_CFLAGS
	instead of CFLAGS.
	* build-with-cxx.m4 (GDB_AC_BUILD_WITH_CXX): Set and AC_SUBST
	COMPILER_CFLAGS.
	* configure: Regenerate.

gdb/gdbserver/ChangeLog:
2016-01-21  Pedro Alves  <palves@redhat.com>

	* Makefile.in (COMPILER_CFLAGS, CXXFLAGS): New.
	(INTERNAL_CFLAGS_BASE): Use COMPILER_CFLAGS instead of CFLAGS.
	* configure: Regenerate.

gdb/configure

@@ -731,6 +731,7 @@ MAKE
 CCDEPMODE
 DEPDIR
 am__leading_dot
+COMPILER_CFLAGS
 COMPILER
 INSTALL_STRIP_PROGRAM
 STRIP
@@ -5015,12 +5016,15 @@ fi
 
   if test "$enable_build_with_cxx" = "yes"; then
     COMPILER='$(CXX)'
+    COMPILER_CFLAGS='$(CXXFLAGS)'
    else
     COMPILER='$(CC)'
+    COMPILER_CFLAGS='$(CFLAGS)'
   fi
 
 
 
+
 # Dependency checking.
 rm -rf .tst 2>/dev/null
 mkdir .tst 2>/dev/null