Trim same amount from strings before comparing

If you specified options in an environment variable containing a blank
at the end, then this would not compare equal to the other candidate,
which was stripped, causing the first added argument to drop out! This
is usually the flag for C++11 compliance, causing binary incompatibility
Merge pull request #170 from rolk/170_trimopts

Fix mysterious dropping of flags when specifying CXXFLAGS env.var.