CMake: FFTW Search w/ CMake Install (#2059)

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`

cmake/dependencies/FFT.cmake

@@ -16,17 +16,15 @@ if(WarpX_PSATD)
         endif()
     endfunction()
 
-    # Check if the PkgConfig target location has an _omp library, e.g.,
-    # libfftw3(f)_omp.a shipped and if yes, set the WarpX_FFTW_OMP=1 define.
+    # Check if the found FFTW install location has an _omp library, e.g.,
+    # libfftw3(f)_omp.(a|so) shipped and if yes, set the WarpX_FFTW_OMP=1 define.
     #
     function(fftw_check_omp library_paths fftw_precision_suffix)
-        if(WarpX_FFTW_IGNORE_OMP)
-            fftw_add_define(FALSE)
-            return()
-        endif()
-
         find_library(HAS_FFTW_OMP_LIB fftw3${fftw_precision_suffix}_omp
             PATHS ${library_paths}
+            # this is intentional, so we don't mix different FFTW installs
+            # and only check what is in the location hinted by the
+            # "library_paths" variable
             NO_DEFAULT_PATH
             NO_PACKAGE_ROOT_PATH
             NO_CMAKE_PATH
@@ -88,9 +86,17 @@ if(WarpX_PSATD)
 
         if(WarpX_FFTW_SEARCH STREQUAL CMAKE)
             find_package(FFTW3${HFFTWp} CONFIG REQUIRED)
+            set(WarpX_FFTW_LIBRARY_DIRS "${FFTW3${HFFTWp}_LIBRARY_DIRS}")
+            message(STATUS "Found FFTW: ${FFTW3${HFFTWp}_DIR} (found version \"${FFTW3${HFFTWp}_VERSION}\")")
         else()
             find_package(PkgConfig REQUIRED QUIET)
             pkg_check_modules(fftw3${HFFTWp} REQUIRED IMPORTED_TARGET fftw3${HFFTWp})
+            message(STATUS "Found FFTW: ${fftw3${HFFTWp}_PREFIX}")
+            if(fftw3${HFFTWp}_LIBRARY_DIRS)
+                set(WarpX_FFTW_LIBRARY_DIRS "${fftw3${HFFTWp}_LIBRARY_DIRS}")
+            else()
+                set(WarpX_FFTW_LIBRARY_DIRS "${fftw3${HFFTWp}_LIBDIR}")
+            endif()
         endif()
     endif()
 
@@ -101,25 +107,19 @@ if(WarpX_PSATD)
     elseif(WarpX_COMPUTE STREQUAL HIP)
         make_third_party_includes_system(roc::rocfft FFT)
     else()
-        if(FFTW3_FOUND)
-            # subtargets: fftw3(p), fftw3(p)_threads, fftw3(p)_omp
-            if(WarpX_COMPUTE STREQUAL OMP AND
-               TARGET FFTW3::fftw3${HFFTWp}_omp AND
-               NOT WarpX_FFTW_IGNORE_OMP)
-                make_third_party_includes_system(FFTW3::fftw3${HFFTWp}_omp FFT)
-                fftw_add_define(TRUE)
-            else()
-                make_third_party_includes_system(FFTW3::fftw3${HFFTWp} FFT)
-                fftw_add_define(FALSE)
-            endif()
+        if(WarpX_FFTW_SEARCH STREQUAL CMAKE)
+            make_third_party_includes_system(FFTW3::fftw3${HFFTWp} FFT)
         else()
             make_third_party_includes_system(PkgConfig::fftw3${HFFTWp} FFT)
-            if(WarpX_COMPUTE STREQUAL OMP AND
-               NOT WarpX_FFTW_IGNORE_OMP)
-                fftw_check_omp("${fftw3${HFFTWp}_LIBRARY_DIRS}" "${HFFTWp}")
+        endif()
+        if(WarpX_COMPUTE STREQUAL OMP)
+            if(WarpX_FFTW_IGNORE_OMP)
+                message(STATUS "FFTW: Requested to IGNORE OpenMP support")
             else()
-                fftw_add_define(FALSE)
+                fftw_check_omp("${WarpX_FFTW_LIBRARY_DIRS}" "${HFFTWp}")
             endif()
+        else()
+            message(STATUS "FFTW: Did NOT search for OpenMP support (WarpX_COMPUTE!=OMP)")
         endif()
     endif()
 endif(WarpX_PSATD)