fix mk/build.mk for HaveLibGmp/HaveFrameworkGMP

Previously if you were to set HaveFrameworkGMP/HaveLibGmp in
mk/build.mk (e.g. on mac os) the settings would be ignored, as
gmp/config.mk is included late in GHC's ghc.mk, meaning autoconf
overrides your settings. Now they ignore the values picked up by 
configure if you set these variables.

gmp/config.mk.in

 
-HaveLibGmp = @HaveLibGmp@
-HaveFrameworkGMP = @HaveFrameworkGMP@
+ifeq ($(HaveLibGmp),"")
+ HaveLibGmp = @HaveLibGmp@
+endif
 
+ifeq ($(HaveFrameworkGMP),"")
+ HaveFrameworkGMP = @HaveFrameworkGMP@
+endif