check if stxxl compiles with or without libgomp, fixes #1361

cmake/check_stxxl.cmake

@@ -0,0 +1,24 @@
+INCLUDE (CheckCXXSourceCompiles)
+unset(STXXL_WORKS CACHE)
+set (STXXL_CHECK_SRC "#include <stxxl/vector>\n int main() { stxxl::vector<int> vec; return 0;}")
+set (CMAKE_TRY_COMPILE_CONFIGURATION ${CMAKE_BUILD_TYPE})
+set (CMAKE_REQUIRED_INCLUDES "${STXXL_INCLUDE_DIR}")
+set (CMAKE_REQUIRED_LIBRARIES "${STXXL_LIBRARY}")
+
+CHECK_CXX_SOURCE_COMPILES("${STXXL_CHECK_SRC}" STXXL_WORKS)
+
+if(STXXL_WORKS)
+  message(STATUS "STXXL can be used without linking against libgomp")
+else()
+  unset(STXXL_WORKS)
+  message(STATUS "STXXL failed without libgomp, retrying ..")
+  set(CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES} -lgomp)
+
+  CHECK_CXX_SOURCE_COMPILES("${STXXL_CHECK_SRC}" STXXL_WORKS)
+
+  if (STXXL_WORKS)
+    target_link_libraries(osrm-extract gomp)
+  else()
+    message(FATAL "STXXL failed failed, libgomp missing?")
+  endif()
+endif()