Require a C99 compiler, not a C11 compiler and improve the C99 detection logic.

configure.ac

 AC_PROG_LIBTOOL
 
 dnl ================================================================
-dnl C11 compiler checks.
+dnl C99 compiler checks.
 dnl ================================================================
 
 AC_LANG_PUSH(C)
-AC_MSG_CHECKING([if $CC supports C11 without any flags])
+AC_MSG_CHECKING([if $CC supports C99 without any flags])
 AC_COMPILE_IFELSE(
 	[AC_LANG_PROGRAM(
-		[[// test]],
+		[[static inline int c99_restrict(int *restrict c99_restrict) {}]],
 		[[]])],
 	[AC_MSG_RESULT([yes])
-	 have_c11_native=yes],
+	 have_c99_native=yes],
 	[AC_MSG_RESULT([no])
-	 have_c11_native=no])
+	 have_c99_native=no])
 AC_LANG_POP(C)
 
 AC_LANG_PUSH(C)
 TEMP_CFLAGS="$CFLAGS"
-CFLAGS="$CFLAGS -std=c11"
-AC_MSG_CHECKING([if $CC supports C11 with the -std=c11 flag])
+CFLAGS="$CFLAGS -std=c99"
+AC_MSG_CHECKING([if $CC supports C99 with the -std=c99 flag])
 AC_COMPILE_IFELSE(
 	[AC_LANG_PROGRAM(
-		[[// test]],
+		[[static inline int c99_restrict(int *restrict c99_restrict) {}]],
 		[[]])],
 	[AC_MSG_RESULT([yes])
-	 have_c11_c11=yes],
+	 have_c99_c99=yes],
 	[AC_MSG_RESULT([no])
-	 have_c11_c11=no])
+	 have_c99_c99=no])
 CFLAGS="$TEMP_CFLAGS"
 AC_LANG_POP(C)
 
-AC_MSG_CHECKING([if $CC supports C11])
-if test "$have_c11_c11" = yes ; then
-	AC_MSG_RESULT([-std=c11])
-	CFLAGS="$CFLAGS -std=c11"
-elif test "$have_c11_native" = yes ; then
+AC_MSG_CHECKING([if $CC supports C99])
+if test "$have_c99_c99" = yes ; then
+	AC_MSG_RESULT([-std=c99])
+	CFLAGS="$CFLAGS -std=c99"
+elif test "$have_c99_native" = yes ; then
 	AC_MSG_RESULT([yes])
 else
 	AC_MSG_RESULT([no])
-	AC_MSG_ERROR([C11 is not supported by $CC.])
+	AC_MSG_ERROR([C99 is not supported by $CC.])
 fi
 
-AC_C_INLINE
-AC_C_RESTRICT
-
 dnl ================================================================
 dnl library checks.
 dnl ================================================================