makeguids: Ensure compatibility with other libcs

The musl libc does not provide __bswap_constant_XX.
If <endian.h> does not provide these macros, use our own.

This fixes issue #84.

Author: awilfox

src/makeguids.c

@@ -152,6 +152,15 @@ main(int argc, char *argv[])
 	fprintf(symout, "#include <efivar/efivar.h>\n");
 	fprintf(symout, "#include <endian.h>\n");
 	fprintf(symout, """\n\
+#ifndef __bswap_constant_16\n\
+#define __bswap_constant_16(x)\\\n\
+	((unsigned short int) ((((x) >> 8) & 0xff) | (((x) & 0xff) << 8)))\n\
+#endif\n\
+#ifndef __bswap_constant_32\n\
+#define __bswap_constant_32(x)\\\n\
+	((((x) & 0xff000000) >> 24) | (((x) & 0x00ff0000) >>  8) | \\\n\
+	 (((x) & 0x0000ff00) <<  8) | (((x) & 0x000000ff) << 24))\n\
+#endif\n\
 #if BYTE_ORDER == BIG_ENDIAN\n\
 #define cpu_to_be32(n) (n)\n\
 #define cpu_to_be16(n) (n)\n\