load OpenMP targets if present in {BLAS,LAPACK}_LIBRARIES

borrows ValeevGroup/kit-cmake#20

Author: evaleev

CMakeLists.txt

@@ -198,6 +198,9 @@ endif()
 if (TARGET vrg-build-external-projects)
   add_dependencies(ttg-imported-packages-build vrg-build-external-projects)
 endif()
+# {PaRSEC,MADNESS} rely on usability of {BLAS,LAPACK}_LIBRARIES
+include(SanitizeLinalgLibraries)
+SanitizeLinalgLibraries()
 # PaRSEC
 include(FindOrFetchPARSEC)
 if (TARGET PaRSEC::parsec)

cmake/modules/SanitizeLinalgLibraries.cmake

@@ -0,0 +1,24 @@
+# SPDX-License-Identifier: BSD-3-Clause
+
+# {BLAS,LAPACK}_LIBRARIES may include IMPORTED targets such as OpenMP::OpenMP_C
+# these targets are usually exist when these variables are "discovered", but they may not if they are already in CACHE
+
+include(CMakeFindDependencyMacro)
+include(CheckLanguage)
+
+function(SanitizeLinalgLibraries)
+    if (BLAS_LIBRARIES OR LAPACK_LIBRARIES)
+        # look for OpenMP
+        foreach (lang IN ITEMS C CXX Fortran)
+            if (BLAS_LIBRARIES MATCHES OpenMP::OpenMP_${lang} OR LAPACK_LIBRARIES MATCHES OpenMP::OpenMP_${lang})
+                check_language(${lang})
+                if(CMAKE_${lang}_COMPILER)
+                    enable_language(${lang})
+                    find_dependency(OpenMP)
+                else()
+                    message(FATAL_ERROR "No support for LANGUAGE ${lang} but {BLAS,LAPACK}_LIBRARIES contains IMPORTED target OpenMP::OpenMP_${lang} that requires it")
+                endif()
+            endif()
+        endforeach()
+    endif()
+endfunction()