Code review comment for lp://qastaging/~alan-griffiths/mir/no-cmake-extras

Pre-existing problem.

11:03:52 ../../../i386-linux-gnu/crt1.o(.text+0x18): error: undefined reference to 'main'
11:03:52 collect2: error: ld returned 1 exit status
11:03:52 tests/performance-tests/CMakeFiles/mir_compositor_performance_test.dir/build.make:98: recipe for target 'bin/mir_compositor_performance_test.bin' failed
11:03:52 make[3]: *** [bin/mir_compositor_performance_test.bin] Error 1

This is racy:

target_link_libraries(mir_compositor_performance_test
  mir_system_performance_test
  ${GTEST_BOTH_LIBRARIES}
)

With no dependency on the gtest target that builds GTEST_BOTH_LIBRARIES