cmake: Specify fftw include/link directories to fix OSX build

Linux build fills FFTW_INCLUDEDIR while OSX build fills FFTW_INCLUDE_DIRS,
so both are included.

Proper future solution will be to fix things to FFTW_LIBRARIES contains an
absolute path

CMakeLists.txt

@@ -35,7 +35,16 @@ list(APPEND inspectrum_sources
 INCLUDE(FindPkgConfig)
 find_package(Qt5Widgets REQUIRED)
 pkg_check_modules(FFTW REQUIRED fftw3f)
-include_directories(${QT_INCLUDES} ${FFTW_INCLUDEDIR})
+
+include_directories(
+  ${QT_INCLUDES}
+  ${FFTW_INCLUDEDIR}
+  ${FFTW_INCLUDE_DIRS}
+)
+
+link_directories(
+  ${FFTW_LIBRARY_DIRS}
+)
 
 add_executable(inspectrum ${inspectrum_sources})
 qt5_use_modules(inspectrum Widgets)