[CMake] Add hack to find LLVM's OpenMP with GCC (#984)

This should replace #960.

It worked on my Ubuntu for both Clang and GCC.

@dbabokin please check that this works for you, too.

Fixes #959.

Author: rengolin

cmake/modules/openmp.cmake

@@ -1,5 +1,19 @@
 option(USE_OpenMP "Use OpenMP" ON)
 
+# We don't want GOMP because its performance sinks for large core count, so we force libomp
+# This finds the library path from the system's clang for OpenMP
+#
+# On Fedora, it's at the same place as others, so we don't need to look elsewhere
+# On Ubuntu, it's in /usr/lib/llvm-${version}, so find_package finds GOMP for GCC instead.
+execute_process (
+    COMMAND bash -c "for lib in $(clang -lomp -### 2>&1); do echo $lib | grep -o \"\\/.*llvm.*\\w\"; done"
+    OUTPUT_VARIABLE LLVM_OMP_PATH
+)
+# Only if we found an "llvm" path that we need to add
+if (LLVM_OMP_PATH)
+  set(CMAKE_PREFIX_PATH ${LLVM_OMP_PATH})
+endif()
+
 if(USE_OpenMP)
   find_package(OpenMP)
   if(OPENMP_FOUND)