Add configuration of include subdirectories

Add the possibility to search headers in several subdirectories of
/usr/local/include and /usr/include. A possible use case would be for
tcc to search for headers in /usr/local/include/x86_64-linux-gnu,
/usr/local/include, /usr/include/x86_64-linux-gnu and /usr/include in
turn.

tcc.h

@@ -150,6 +150,9 @@ typedef int BOOL;
   #endif
 #endif
 #define CONFIG_TCC_CRT_PREFIX CONFIG_SYSROOT "/usr" CONFIG_TCC_LDDIR
+#ifndef CONFIG_TCC_INCSUBDIR
+  #define CONFIG_TCC_INCSUBDIR ""
+#endif
 
 #define INCLUDE_STACK_SIZE  32
 #define IFDEF_STACK_SIZE    64