Fix disabling of -Werror in PGAC_PROG_CXX_CXXFLAGS_OPT macro

This was copy-pasted from PGAC_PROG_CC_CFLAGS_OPT, but for the g++
compiler, we need to set ac_cxx_werror rather than ac_c_werror.

The point of this dance with Werror is to detect if the compiler accepts a
flags, but causes warnings, like:

cc1plus: warning: command line option ‘-Wmissing-prototypes’ is valid for C/ObjC but not for C++

We don't want to use such a flag.

config/cxx-compiler.m4

@@ -6,13 +6,13 @@ AC_DEFUN([PGAC_PROG_CXX_CXXFLAGS_OPT],
 [AC_MSG_CHECKING([if $CXX supports $1])
 pgac_save_CXXFLAGS=$CXXFLAGS
 CXXFLAGS="$pgac_save_CXXFLAGS $1"
-ac_save_c_werror_flag=$ac_c_werror_flag
-ac_c_werror_flag=yes
+ac_save_cxx_werror_flag=$ac_cxx_werror_flag
+ac_cxx_werror_flag=yes
 AC_LANG_PUSH([C++])
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM()],
                    AC_MSG_RESULT(yes),
                    [CXXFLAGS="$pgac_save_CXXFLAGS"
                     AC_MSG_RESULT(no)])
 AC_LANG_POP([C++])
-ac_c_werror_flag=$ac_save_c_werror_flag
+ac_cxx_werror_flag=$ac_save_cxx_werror_flag
 ])# PGAC_PROG_CXX_CXXFLAGS_OPT

configure

@@ -4990,8 +4990,8 @@ ac_compiler_gnu=$ac_cv_cxx_compiler_gnu
 $as_echo_n "checking if $CXX supports -fno-aggressive-loop-optimizations... " >&6; }
 pgac_save_CXXFLAGS=$CXXFLAGS
 CXXFLAGS="$pgac_save_CXXFLAGS -fno-aggressive-loop-optimizations"
-ac_save_c_werror_flag=$ac_c_werror_flag
-ac_c_werror_flag=yes
+ac_save_cxx_werror_flag=$ac_cxx_werror_flag
+ac_cxx_werror_flag=yes
 ac_ext=cpp
 ac_cpp='$CXXCPP $CPPFLAGS'
 ac_compile='$CXX -c $CXXFLAGS $CPPFLAGS conftest.$ac_ext >&5'
@@ -5024,7 +5024,7 @@ ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
 ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
 ac_compiler_gnu=$ac_cv_c_compiler_gnu
 
-ac_c_werror_flag=$ac_save_c_werror_flag
+ac_cxx_werror_flag=$ac_save_cxx_werror_flag
 
 
   # Silence compiler warnings that you get with modern versions of GCC.