chore(build): replace -Wc++-compat' with -x c++'

As of this commit, a build with CFLAGS="-x c++" succeeds; ie: all source
files in the tree are both valid C++ source code AND valid C code.

Convert wcpp-compat distcheck builds to actually build with C++, to
prevent future pull requests from breaking C++ compatibility going
forward.

stack-info: PR: #578, branch: aws-nslick/stack/25
Signed-off-by: Nicholas Sielicki <nslick@amazon.com>

configure.ac

@@ -144,14 +144,14 @@ AC_DEFINE_UNQUOTED([OFI_NCCL_TRACE], [${trace}], [Defined to 1 unit test output
 AC_ARG_ENABLE([picky-compiler],
    [AS_HELP_STRING([--disable-picky-compiler], [Disable adding picky compiler flags.])])
 AS_IF([test "${enable_picky_compiler}" != "no"],
-      [picky_compiler_flags="-Wall -Wc++-compat -Wextra -Wno-unused-parameter"
+      [picky_compiler_flags="-Wall -Wextra -Wno-unused-parameter"
        AC_MSG_NOTICE([Adding ${picky_compiler_flags} to CFLAGS.])
        CFLAGS="${CFLAGS} ${picky_compiler_flags}"
        AS_UNSET([picky_compiler_flags])])
 
 AC_ARG_ENABLE([werror],
    [AS_HELP_STRING([--enable-werror], [Enable setting -Werror.  Off by default, unless building from Git tree.])])
-werror_flags="-Werror -Wno-error=unused-parameter -Wno-error=missing-field-initializers"
+werror_flags="-Werror -Wno-error=unused-parameter -Wno-error=missing-field-initializers -x c++"
 AS_IF([test -d "${srcdir}/.git" -a -z "${enable_werror}"],
       [AC_MSG_NOTICE([Found .git directory.  Adding ${werror_flags} to CFLAGS.])
        CFLAGS="${CFLAGS} ${werror_flags}"],