Make it workable on OpenBSD

- Additional LMMS_BUILD flag.
- Disallow on plugins -Wl,-no-undefined which triggers undefined references.
- Make sure X11 headers are found.

Lib ossaudio is needed only for OpenBSD

redundant expression removal

simplify condition for detection OS 'kind'

seems the last commit brought an issue on OSx travis test ....

src/CMakeLists.txt

@@ -106,6 +106,10 @@ IF(LMMS_BUILD_APPLE)
 	SET(EXTRA_LIBRARIES "-framework CoreMIDI")
 ENDIF()
 
+if(LMMS_HAVE_OSS AND LMMS_BUILD_OPENBSD)
+    SET(EXTRA_LIBRARIES "-lossaudio")
+endif()
+
 SET(LMMS_REQUIRED_LIBS
 	${CMAKE_THREAD_LIBS_INIT}
 	${QT_LIBRARIES}

src/lmmsconfig.h.in

@@ -2,6 +2,7 @@
 #cmakedefine LMMS_BUILD_WIN32
 #cmakedefine LMMS_BUILD_WIN64
 #cmakedefine LMMS_BUILD_APPLE
+#cmakedefine LMMS_BUILD_OPENBSD
 #cmakedefine LMMS_BUILD_HAIKU
 
 #cmakedefine LMMS_HOST_X86