[#392] Don't install tests
[#392] Don't install tests

'make install' should install the GSL library (ie. the headers),
not the tests.

It still installs the UnitTest++ headers, but that will be more
complex to fix, as GSL just imports UnitTest++ as a git submodule,
and the install command propagates down to UnitTest++'s
CMakeLists.txt.
kernhanda/GSL,bashrc-real/GSL,Josaphat/GSL,neilmacintosh/GSL,martinmoene/GSL,garyfurnish/GSL