Update to gtest 1.7.0.

1.6.0 has troubles with clang in c++11 mode, it seems.
update Mac OS X install instructions, part 2
PS to previous commit: also updated 4ti2 to the latest version
Merge branch 'master' of https://github.com/Macaulay2/M2
update Mac OS X install instructions, part 3
Merge branch 'master' of github.com:Macaulay2/M2
oops, update the URL!
fixed accidental duplicate targets
Merge branch 'master' of https://github.com/Macaulay2/M2