configure: assume LD=CXX unless explicitly set
configure: assume LD=CXX unless explicitly set

Previously, using a different compiler (e.g. clang) would require
setting both the CXX and LD environment variables to point to the
relevant executable. This commit sets LD to CXX unless LD is itself set
explicitly. This is required for TravisCI since it only sets the CXX
variable.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3