Let Autoconf handle enabling C and POSIX extensions
The comment on src/common/compat.c:18 suggests using AC_USE_SYSTEM_EXTENSIONS. The attached patch does just that.
Defining the preprocessor macros globally ensures the extensions are always turned on. Running GCC in pedantic mode tells me that there are instances where the code assumes the extensions are enabled while missing the proper preprocessor macros. FWICT it currently just works because GCC and its headers turn their own extensions on by default.
The comment suggestion about maybe removing the environ configure check has not been researched.
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