CMake: detect OpenMP and use if available.

This is a missing part of commit b4e1ce44.

CMakeLists.txt

@@ -96,6 +96,14 @@ if(CMAKE_SYSTEM_PROCESSOR STREQUAL "i686" OR CMAKE_SYSTEM_PROCESSOR STREQUAL "X8
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${FLOAT_FLAGS}")
 endif()
 
+# We use OpenMP to speed up some geometric operations, but it's optional.
+include(FindOpenMP)
+if(OpenMP_FOUND)
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
+else()
+    message(WARNING "OpenMP not found, geometric operations will be single-threaded")
+endif()
+
 if(APPLE OR CMAKE_SYSTEM_NAME STREQUAL "FreeBSD")
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -stdlib=libc++")
 endif()

src/CMakeLists.txt

@@ -224,6 +224,7 @@ add_dependencies(solvespace-core
     q3d_header)
 
 target_link_libraries(solvespace-core
+    ${OpenMP_CXX_LIBRARIES}
     dxfrw
     ${util_LIBRARIES}
     ${ZLIB_LIBRARY}