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This makes it easier to use GTest in projects that build with the
-Wmissing-declarations warning. This fixes the warning in headers and
source files, though not GTest's own tests as it is rather noisy there.

Per the review comments.

Without the directive embedding googletest into third_party breaks
any //... target.

This is part (hopefully all) of the fixes for #776.  The top-level
configure.ac configures googletest first and then googlemock.
With this changes it is possible to embed googletest into another
project that uses autoconf.  For an example (though it is WIP), see
the commits (and soon PR) referenced from google/protobuf#236.

change static_cast to ImplicitCast_ for consitency
fixes for building with path names containing spaces

If the user's cmakelists.txt first look for threads using
find_package(Threads), then set(gtest_disable_pthreads ON),
and then include googletest. GoogleTest will not look for
threads. But since they have already been found before in
user's cmakelists, it will use them regardless.

This helped me fix build issue in darktable-org/rawspeed
on windows/MSYS2, even though there are threads, and they
are usable, googletest build was failing with issues
about AutoHandle. I was first looking for threads, and only
then including googletest, so no matter the value of
gtest_disable_pthreads, it failed.

The other obvious solution is for user to first include
googletest, and only then look for threads by himself.

VS2010 solution only to simplify old users (who used these solutions) upgrading to new gtest/gmock, new users should use CMake generated solutions. VS2010 solution can be opened in any new VS.

Slightly better names and cleaner tests.
Please review

Typos, punctuation & broken links

Modify library install destinations to install .dll's to the correct
location (`bin`, not `lib`), and to install other artifacts to the
correct platform-dependent location by using GNUInstallDirs. This is
required for some distributions (e.g. Fedora) and will fix an issue that
otherwise requires those distributions to patch the upstream sources.
Also, add options to suppress installation, which may be useful for
projects that embed Google Test.

Since Google Test is trying to support archaic versions of CMake, a
brain-dead fallback (which requires that the user set either LIB_SUFFIX
or CMAKE_INSTALL_LIBDIR themselves) is included for versions that
predate GNUInstallDirs.

Fixes #1161.
Co-Authored-By: d3x0r <d3x0r@users.noreply.github.com>