maint: avoid new automake warning about AM_PROG_CC_STDC

* configure.ac (AM_PROG_CC_STDC): Stop using this macro.
It provokes warnings from newer automake and is superseded by
autoconf's AC_PROG_CC, which we're already using.

configure.ac

@@ -80,7 +80,6 @@ dnl Checks for C compiler.
 AC_PROG_CC
 AC_PROG_INSTALL
 AC_PROG_CPP
-AM_PROG_CC_STDC
 
 gl_EARLY
 gl_INIT