make: use -std=c99 as default

Makefile.cflags

 # Test if the input language was specified externally.
-# Otherwise test if the compiler unterstands the "-std=gnu99" flag, and use it if so.
 # Otherwise test if the compiler unterstands the "-std=c99" flag, and use it if so.
 ifeq ($(filter -std=%,$(CFLAGS)),)
-  ifeq ($(shell $(CC) -std=gnu99 -E - 2>/dev/null >/dev/null </dev/null ; echo $$?),0)
-    CFLAGS += -std=gnu99
-  else
-    ifeq ($(shell $(CC) -std=c99 -E - 2>/dev/null >/dev/null </dev/null ; echo $$?),0)
-      CFLAGS += -std=c99
-    endif
+  ifeq ($(shell $(CC) -std=c99 -E - 2>/dev/null >/dev/null </dev/null ; echo $$?),0)
+    CFLAGS += -std=c99
   endif
 endif