CMake: FFTW Search w/ CMake Install

[ax3l]

This adds support for FFTW search with OpenMP support if FFTW was installed with CMake. This is, for instance, the case with EasyBuild based installs on Juelich's JUWELS Booster cluster.

Fixes:

• the FFTW install does NOT define CMake targets for their sub-targets, so we need to manually find and link the _omp lib
• the library dir hint with CMake was empty because of a differing spelling (upper vs lowercase of FFTW from pkg-config check module)

Tested:

• installed with pkg-config, SP and DP, picked up with *.pc
• installed with CMake, SP and DP, picked up with *config.cmake
• installed with CMake, SP and DP, picked up with *.pc

System check:

• still need to figure our why the pkg-config module set fftw3${HFFTWp}_LIBRARY_DIRS is unset on JUWELS (but fftw3${HFFTWp}_PREFIX is set

Follow-up to #541

• Small enough (< few 100s of lines), otherwise it should probably be split into smaller PRs
• Tested (describe the tests in the PR description)
• Runs on GPU (basic: the code compiles and run well with the new module)
• Contains an automated test (checksum and/or comparison with theory)
• Documented: all elements (classes and their members, functions, namespaces, etc.) are documented
• Constified (All that can be const is const)
• Code is clean (no unwanted comments, )
• Style and code conventions are respected at the bottom of https://github.com/Hi-PACE/hipace
• Proper label and GitHub project, if applicable

[ax3l]

@MaxThevenet @SeverinDiederichs now also tested on JUWELS and updated accordingly :)

[MaxThevenet]

Awesome, thanks for this PR!!

[ax3l]

😊