Older CMake versions, in particular 2.8, don't seem to correctly handle
interface include directories. This causes failures when building the
tests. Additionally, older CMake versions use a different library install
directory than expected (i.e. they use lib/<target-triple>). This caused
certain tests to fail to link.
This patch fixes both those issues. The first by manually adding the
correct include directory when building the tests. The second by specifying
the library output directory when configuring the GTest build.
We're propagating extra warning flags to the gtest build, which
can cause it to fail. This patch prevents passing "-Wextra" to
gtest, since the library itself doesn't test with that flag.
When users satisfy the GTest dependancy by placing a googletest
directory in the project, the targets from GTest and GMock incorrectly
get installed along side this library. We shouldn't be installing
our test dependancies.
This patch forces the options that control installation for googletest
to OFF.
* Add support for GTest based unit tests.
As Dominic and I have previously discussed, there is some
need/desire to improve the testing situation in Google Benchmark.
One step to fixing this problem is to make it easier to write
unit tests by adding support for GTest, which is what this patch does.
By default it looks for an installed version of GTest. However the
user can specify -DBENCHMARK_BUILD_EXTERNAL_GTEST=ON to instead
download, build, and use copy of gtest from source. This is
quite useful when Benchmark is being built in non-standard configurations,
such as against libc++ or in 32 bit mode.
