Add -I$(GHC_INCLUDE_DIR) to SRC_MKDEPENDHS_OPTS; ghc-4.0x
needs this since 'mkdependHS' also chases #includes.

Not an issue with ghc-5.xx's dep-generator, since it invokes
CPP prior to gathering up the imports.

This should bring ghc-4.08-based nightly builds back to life.