Require C99 to build Guile

* configure.ac: Refactor check to require C99, not just try to get C99.

configure.ac

@@ -74,7 +74,11 @@ if test "$GCC" = yes; then
   AC_MSG_RESULT([-std=gnu11])
   CC="$CC -std=gnu11"
 else
-  AC_MSG_RESULT([assuming $CC supports C11 by default])
+  dnl Guile requires C99 or later.
+  AC_PROG_CC_C99
+  if test "$ac_cv_prog_cc_c99" = "xno"; then
+    AC_MSG_ERROR([Support for C99 required but not found.])
+  fi
 fi
 
 gl_EARLY
@@ -94,9 +98,6 @@ AC_DEFINE([GNULIB_LOCK], [1],
   [Define to allow Gnulib modules to use Guile's locks.])
 
 
-dnl Guile needs C99 or later.
-gl_PROG_CC_C99
-
 # for per-target cflags in the libguile subdir
 AM_PROG_CC_C_O