_AC_PROG_CC_C99 was using the wrong test program.
Fixes #110396, reported anonymously.
* lib/autoconf/c.m4 (_AC_PROG_CC_C99): Use the C99 test program, not
the C89 test program.
# option to output variable CC to make it so.
AC_DEFUN([_AC_PROG_CC_C99],
[AC_REQUIRE([_AC_C_C99_TEST_PROGRAM])]dnl
-[_AC_C_STD_TRY([c99], [ac_c_conftest_c89_program],
+[_AC_C_STD_TRY([c99], [ac_c_conftest_c99_program],
dnl Try
dnl GCC -std=gnu99 (unused restrictive modes: -std=c99 -std=iso9899:1999)
dnl IBM XL C -qlanglvl=extc1x (V12.1; does not pass C11 test)