Support Ninja Multi-Config generator

[K-ballo]

Trying to use Ninja Multi-Config generator on linux results in errors:

[cmake] CMake Error in CMakeLists.txt:
[cmake]   Evaluation file to be written multiple times with different content.  This
[cmake]   is generally caused by the content evaluating the configuration type,
[cmake]   language, or location of object files:
[cmake] 
[cmake]    /home/k/hpx/build/lib/pkgconfig/hpx_application_debug.pc

The culprit is this line https://github.com/STEllAR-GROUP/hpx/blob/15afc8be41b0668584dfafe2d8cf6ffb75dfb715/cmake/HPX_GeneratePackageUtils.cmake#L399

[hkaiser]

The easiest workaround would be to diable support for pkg_config by using -DHPX_WITH_PKGCONFIG=Off.