Fix ODR violation of gtest global variables in gmock

[igorsugak]

When both gtest and gmock are DSOs configured by cmake, and are dynamically
linked into a program, the program crashes with a double free error upon exit,
if the test program is invoked with at least one gtest flag specified,
or if there is a death test in the program. This is due to ODR violation
of global std::string variables (holding command line arguments) defined in
gtest but duplicated in gmock, because gtest is linked into gmock.
This updates cmake configuration for gmock to remove gtest from gmock
libraries. Notice that the gmock configure script also doesn't link gtest into gmock.
This also removes gmock from gmock_main to be consistent with gtest and
gtest_main.

Tested by running googletest unit tests as well by running tests in a
larger codebase that uses googletest (both DSO and .a) as a testing framework
on a linux system with gcc-4.9 and gcc-5.4.

Potentially also fixes some bugs mentioned in #930

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[igorsugak]

Looks like this broke googletest on windows and build on linux when cmake configures from googlemock (not root) directory. I'll update my patch to handle this.

[KiNgMaR]

I have run into this error on Linux and can confirm that the patch fixes it.

[gennadiycivil]

Both Travis and AppVeyor failed. Closing