cmake: add USE_SYSTEM_* options; support find_package(pthreadpool)

[ConnorBaker]

Similar to the work done in pytorch/pytorch#37137, this adds the following CMake options:

• USE_SYSTEM_LIBS
• USE_SYSTEM_GOOGLEBENCHMARK
• USE_SYSTEM_GOOGLETEST
• USE_SYSTEM_FXDIV

This is particularly useful in the context of Nix, where we can build these libraries once and then re-use them elsewhere to avoid rebuilding vendors dependencies.

Additionally, adds a CMake configuration file to make it possible for CMake-based projects to use find_package to consume this library.

[ConnorBaker]

Updated to reflect comment from pytorch/cpuinfo#153 (comment).

@Maratyszcza if you have the time, would you mind reviewing this?

[petrhosek]

I'd be interested in seeing this merged as well since we need this change for the https://github.com/google/ml-compiler-opt project. @Maratyszcza would it be possible to take a look?

[petrhosek]

I believe this also needs the following prior to this line to make sure FXdiv is present:

Suggested change

	@PACKAGE_INIT@
	include(CMakeFindDependencyMacro)
	find_dependency(FXdiv)
	@PACKAGE_INIT@

[BwL1289]

Looks like this is stale, but would like to see DPTHREADPOOL_USE_CPUINFO added as an option in cmake. If im not mistaken, its only supported in bazel currently.

[BwL1289]

@Maratyszcza any chance we can merge this?

@ConnorBaker sorry for all of the noise. I forked master and manually added your changes here: https://github.com/eugo-inc/pthreadpool-use-system-libs/commits/master/cmake so this is up to date