stb_c_lexer: don't define default macro definitions on non-implementation includes

stb_c_lexer.h

@@ -42,6 +42,7 @@
 //
 //   See end of file for license information.
 
+#ifdef STB_C_LEXER_IMPLEMENTATION
 #ifndef STB_C_LEXER_DEFINITIONS
 // to change the default parsing rules, copy the following lines
 // into your C/C++ file *before* including this, and then replace
@@ -96,6 +97,7 @@
 #define STB_C_LEXER_DEFINITIONS         // This line prevents the header file from replacing your definitions
 // --END--
 
+#endif
 #endif
 
 #ifndef INCLUDE_STB_C_LEXER_H