Add JS_VERSION_MAJOR/MINOR/PATCH defines to mujs.h

A macro JS_CHECKVERSION(major, minor, patch) can be used to test the
version if your code depends on API features added in a given version.

#if JS_CHECKVERSION(1, 2, 0)
    ... use new API ...
#else
    ... don't use new API ...
#endif

main.c

@@ -356,6 +356,8 @@ main(int argc, char **argv)
 	}
 
 	if (interactive) {
+		printf("Welcome to MuJS %d.%d.%d.\n",
+			JS_VERSION_MAJOR, JS_VERSION_MINOR, JS_VERSION_PATCH);
 		if (isatty(0)) {
 			using_history();
 			rl_bind_key('\t', rl_insert);

mujs.h

@@ -7,6 +7,13 @@
 extern "C" {
 #endif
 
+#define JS_VERSION_MAJOR 1
+#define JS_VERSION_MINOR 2
+#define JS_VERSION_PATCH 0
+
+#define JS_VERSION (JS_VERSION_MAJOR * 10000 + JS_VERSION_MINOR * 100 + JS_VERSION_PATCH)
+#define JS_CHECKVERSION(x,y,z) (JS_VERSION >= ((x) * 10000 + (y) * 100 + (z)))
+
 /* noreturn is a GCC extension */
 #ifdef __GNUC__
 #define JS_NORETURN __attribute__((noreturn))