Clean up the gcc package (#3511)

* Clean up the gcc package

* Add preliminary testing support

* Older versions of GCC do not depend on MPC

* DejaGnu 1.4.4 cannot be built in parallel

* Fix GCC test dir

* Add two more deps required to run the testsuite

var/spack/repos/builtin/packages/dejagnu/package.py

@@ -37,3 +37,7 @@ class Dejagnu(AutotoolsPackage):
 
     depends_on('expect')
     depends_on('tcl@8.5:')
+
+    # DejaGnu 1.4.4 cannot be built in parallel
+    # `make check` also fails but this can be ignored
+    parallel = False