Fold AC_C_STRINGIZE into AC_PROG_CC.

Fold AC_C_STRINGIZE into AC_PROG_CC.

Another of the last few uses of AC_EGREP_CPP is to check for the ISO C
“stringification” operator.  As this is a feature of C89, let’s fold
the test into the extensive C89 tests we already have, and make
AC_C_STRINGIZE just lean on AC_PROG_CC, in the same way
AC_C_PROTOTYPES does.

* lib/autoconf/c.m4 (_AC_C_C89_TEST_GLOBALS): Add test of preprocessor
  stringification and token pasting.
  (AC_C_STRINGIZE): Just check ac_prog_cc_stdc.