Restore -lomp for OSX

Travis build indicates it does not recognize -fopenmp.

.travis.yml

@@ -18,7 +18,6 @@ matrix:
             - g++-7
             - gcc-7
             - libfftw3-dev
-            - libomp-dev
             - qt5-default
             - libusb-1.0-0-dev
       env:

Desktop_Interface/Labrador.pro

@@ -360,7 +360,8 @@ DISTFILES += \
     build_android/package_source/res/xml/device_filter.xml
 
 # Vincenzo added these to get multithreading on Unix fftw
-unix: LIBS += -fopenmp
+unix:!macx: LIBS += -fopenmp
+macx: LIBS += -lomp
 unix: LIBS += -lfftw3f_omp
 unix: LIBS += -lfftw3_threads
 macx: INCLUDEPATH += /usr/local/include