cc/gcc: add option to enable/disable libmudflap

For some scenarii, libmudflap is not very usefull
or can break the build. Make in an optioon that
defaults to 'N' to be on the safe side.

For the core gcc-s, there is absolutely no need
to build libmidflap.

Idea from: Bernhard Pfund <bernhard@chapter7.ch>

 # Default test suite configuration

 # Default DejaGnu configuration

 DG_TOOLNAME = gcc

 DG_TARGET_HOSTNAME = 127.0.0.1

 DG_TARGET_USERNAME = root

 # Default tests

 DG_C_TESTS = 

 DG_CPP_TESTS =