Omit assembler source rules if no asm sources are found
Regenerate gtest-targets.mk using mktargets.sh/py

This simplifies updating all makefiles consistently.
Merge pull request #155 from mstorsjo/update-gtest-makefile

Regenerate gtest-targets.mk using mktargets.sh/py
Merge remote-tracking branch 'upstream/master' into travis.osx