Gmock from source on Debian buster

[sloretz]

This fixes some issues with gmock tests on Debian buster. The gtest rosdep key resolves to libgtest-dev on Debian buster. This package depends on googletest, and also installs gtest headers to /usr/include. The package googletest includes both gtest and gmock, as source files in /usr/src/googletest/. Additionally there is FindGTest.cmake module installed by the package cmake-data. This situation tricks the current logic.

First the search paths here weren't considering that gtest/gmock could be installed in /usr/src. This PR adds those paths.

Second Because GTest in /usr/include is found by the cmake module, the logic sees GTEST_FOUND but not GMOCK_FOUND. The find_package()'d gtest is used gmock is considered unavailable, but gmock would be available if this code tried to find its sources. This PR refactors the logic to the following:

1. If both gtest and gmock are found using find_package(), use that
2. If neither gtest or gmock are found, try to find it them from source and use whatever is found
3. If only 1 of gtest or gmock is found, try to find both from source. If both are found from source, use that. If only one is found from source, use what was found by the find_package() calls.
4. Finally try to create gtest, gtest_main, gmock and gmock_main targets if they don't exist no matter where we're using gtest/gmock from.

[lromor]

Thank you for this patch!

[GladOSkar]

Thanks, works like a charm :)

[dirk-thomas]

Thanks for the patch.