Disable OpenMP.

This is unfortunate, but OpenMP is not available on some toolchains
(most MinGW builds) and, more importantly, breaks the distribution
model we use on Windows, which is a single self-contained executable.

Leave the OpenMP disabled by default everywhere so that we don't have
any second-class platforms where SolveSpace is slower than on first-
class ones.

Fixes #631.

CMakeLists.txt

@@ -96,13 +96,11 @@ 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)
-# No MinGW distribution actually ships an OpenMP runtime, but FindOpenMP detects it anyway.
-if(OpenMP_FOUND AND NOT MINGW)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
-else()
-    message(WARNING "OpenMP not found, geometric operations will be single-threaded")
+if(ENABLE_OPENMP)
+    include(FindOpenMP)
+    if(OpenMP_FOUND)
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
+    endif()
 endif()
 
 if(APPLE OR CMAKE_SYSTEM_NAME STREQUAL "FreeBSD")