AC_PROG_CC: clarify documentation, encourage -qlanglvl=extc1x

AC_PROG_CC: clarify documentation, encourage -qlanglvl=extc1x

* doc/autoconf.texi (C Compiler): Document the intent of AC_PROG_CC
better.  It's not meant to check for strict conformance, only to
get the latest version with extensions.
(Running the Preprocessor, Present But Cannot Be Compiled):
Document new diagnostic wording.
* lib/autoconf/c.m4 (_AC_C_STD_TRY): Change diagnostic wording from
"checking for gcc option to accept ISO C11" to
"checkint for gcc option to enable C11 features", as this better
reflects what is actually happening.
(_AC_PROG_CC_C99): Put -qlanglvl=extc1x here ...
(_AC_PROG_CC_C11): ... rather than here, as it doesn't pass the C11
test with IBM XL C V12.1, and the point is to enable features not to
test for strict conformance.