Update to gtest 1.7.0.

1.6.0 has troubles with clang in c++11 mode, it seems.
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
added newer linux targets to personal makefile
add a .gitignore file for M2/autoconf/
add an option EXECSTACK=no to bypass failing "execstack -c ..." command
add an option EXECSTACK=no, part 2 (oops)
add an option EXECSTACK=no, part 3

now printing an explanation if an error occurs
update autoconf version in our makefile
my makefile
insist on autoconf >= 2.68 now, for the sake of mpir
update .gitgnore
remove obsolete shell script for use with svn
add an option EXECSTACK=no, part 4

improved instructions for user
newer linux makefile target
Merge branch 'master' of https://github.com/Macaulay2/M2