The move to /usr/include/private prefixed paths seems to require a bit more
effort in order to compile programs.
Install the headers to /usr/include/private/g{mock,test}/... and automatically
include /usr/include/private in GTESTS_CXXFLAGS to make compilation seamless. I
will work on the more global problem later with @bdrewery.
The former is from googletest.test.mk, whereas the latter is from plain.test.mk.
As noted in r344328, Kyua will adopt more native GoogleTest support. Thus, it's
more desirable to make the test interface more of an opaque blackbox for the
testcase implementer.