Allow specifying GTest/benchmark locations with -D${packagename}_ROOT

[iskunk]

I have my own builds of the GTest and benchmark libraries that I want to use in the cpuinfo build, but the USE_SYSTEM_LIBS option assumes that these are installed in the default compiler/linker paths, without allowing for an alternate location (e.g. somewhere under $HOME or /opt).

This patch enables the standard *_ROOT convention supported by find_package() for these libraries, i.e. -DGTest_ROOT=/path/to/gtest-prefix and -Dbenchmark_ROOT=/path/to/benchmark-prefix.

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[iskunk]

Updated after confirming that the source-download path can also link against GTest::gtest and the like, thus rendering unnecessary the *_LIB variables.

[iskunk]

Also confirmed that the INCLUDE_DIRECTORIES() directives are not needed.