tccpp: Implement __BASE_FILE__ macro

Like __FILE__ but always refers to the command line file name also
from inside headers.

tests/tcctest.h

@@ -0,0 +1,9 @@
+static inline const char *get_basefile_from_header(void)
+{
+  return __BASE_FILE__;
+}
+
+static inline const char *get_file_from_header(void)
+{
+  return __FILE__;
+}