Makefile: Propagate correct srcdir to gcc-c-api submake

If srcdir=/a/b is defined in the environment or on the make command line, then
srcdir is inherited with the same value in a submake.  However, the use of
srcdir in gcc-c-api/Makefile indicates that the expected value is
/a/b/gcc-c-api.

Fix this by propagating srcdir explicitly on the submake command line.

Author: vries

Makefile

@@ -149,7 +149,7 @@ $(PLUGIN_DSO): $(PLUGIN_OBJECT_FILES) $(LIBGCC_C_API_SO)
 	    -lgcc-c-api -Lgcc-c-api -Wl,-rpath=$(GCCPLUGINS_DIR)
 
 $(LIBGCC_C_API_SO):
-	cd gcc-c-api && make libgcc-c-api.so CC=$(CC)
+	cd gcc-c-api && make libgcc-c-api.so CC=$(CC) $(if $(srcdir),srcdir=$(srcdir)./gcc-c-api/)
 
 $(PLUGIN_OBJECT_GENERATED_FILES): CPPFLAGS+= $(if $(srcdir),-I$(srcdir))