added test case for subdirs in c source files

test/A/Setup.description

@@ -2,7 +2,7 @@ Name: test
 Version: 1.0
 Modules: A, B.A, C
 Exposed-Modules: A
-C-Sources: hello.c
+C-Sources: hello.c, c_src/hello.c
 Extensions: ForeignFunctionInterface
 
 Executable: testA

test/A/c_src/hello.c

+int foo () {return 9;}