basic/log: add a define for path relative to source root

When using build/ directory inside of the source directory:
__FILE__: ../src/test/test-log.c
RELATIVE_SOURCE_PATH: ..
PROJECT_FILE: src/test/test-log.c

When using a build directory outside of the source directory:
__FILE__: ../../../home/zbyszek/src/systemd-work/src/test/test-log.c
RELATIVE_SOURCE_PATH: ../../../home/zbyszek/src/systemd-work
PROJECT_FILE: src/test/test-log.c

src/basic/log.h

@@ -73,6 +73,9 @@ int log_get_max_level_realm(LogRealm realm) _pure_;
  * for the application itself.
  */
 
+assert_cc(STRLEN(__FILE__) > STRLEN(RELATIVE_SOURCE_PATH) + 1)
+#define PROJECT_FILE (__FILE__ + STRLEN(RELATIVE_SOURCE_PATH) + 1)
+
 int log_open(void);
 void log_close(void);
 void log_forget_fds(void);