Add some options for building LLVM in different environments:

--force-configure to force running configure before building.

--extra-llvm-config-flags
--extra-llvm-gcc-config-flags
--extra-gcc-config-flags

Pass additional argument to the various configure invocations.

This also eliminates a default build flavor because explicitly
specifying builds could result in build flavors being run repeatedly.

Finally, turn off fortran builds for the moment because install
appears to be broken.

llvm-svn: 126510