Updates to CMakelists.txt to export target and create pkg-config file, adding meson build file

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Description of pull request

This PR is an update to CMakelists.txt to make it easier to use astro-accelerate as a dependency.

When make install is run:

• An astro-accelerateConfig.cmake file for find_package will be installed to ${CMAKE_INSTALL_PREFIX}/cmake.
• A pkg-config file is generated and installed to ${CMAKE_INSTALL_PREFIX}/lib/pkgconfig.
• Header files are copied to ${CMAKE_INSTALL_PREFIX}/include/aa
• astroaccelerate binary is copied to ${CMAKE_INSTALL_PREFIX}/bin

I also added some lines to quash cmake policy warnings that were popping up.

Interfaces Will this pull request result in a backwards-incompatible interface change: no

Expected semantic version number increment category (Please indicate x.y.z): 1.3.2

Notes

This PR was motivated by my plan to make some utilities (plugins for bifrost) using astro-accelerate. Thought I should to share in case others could benefit. I want to use meson as a build tool -- I don't really know what I'm doing with cmake so my changes may not be the canonical approach!

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Update: I have now added a meson.build file, so the project can be built with meson as well

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thanks again @eli-schwartz!

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TODO: Add generation of aa_version.hpp via Meson.

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DONE: need to change

-join_paths(lib\pkgconfig "\${prefix}" "${CMAKE_INSTALL_LIBDIR}")
+join_paths(CMAKE_INSTALL_LIBDIR "lib/pkgconfig" "${prefix}")

and need to add

set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
include(JoinPaths)

[KAdamek]

Thanks for updating Cmake.