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

[aws-nslick]

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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.

Signed-off-by: Nicholas Sielicki nslick@amazon.com