Only use FITS support if explicitly enabled.

--- luminance-hdr-2.4.0/CMakeLists.txt
+++ luminance-hdr-2.4.0/CMakeLists.txt
@@ -10,6 +10,8 @@
 include(cmake/CompilerSettings.cmake)
 include(cmake/Internationalization.cmake)
 
+OPTION(USE_FITS "Enable FITS support." ON)
+
 # find and setup Qt5 for this project
 
 # Tell CMake to run moc when necessary:
@@ -122,13 +124,15 @@
 INCLUDE_DIRECTORIES(${JPEGLIB_INCLUDE_DIR})
 INCLUDE_DIRECTORIES(${LCMS2_INCLUDE_DIR})
 
-FIND_PACKAGE(CCfits)
-IF(CCFITS_FOUND)
-    INCLUDE_DIRECTORIES(${CCFITS_INCLUDE_DIRS})
-    SET(LIBS ${LIBS} ${CCFITS_LIBRARIES})
-    ADD_DEFINITIONS(-DHAVE_CCFITS)
-ELSE()
-    message(WARNING "CCfits not found! Building without FITS support!")
+IF(USE_FITS)
+    FIND_PACKAGE(CCfits REQUIRED)
+    IF(CCFITS_FOUND)
+        INCLUDE_DIRECTORIES(${CCFITS_INCLUDE_DIRS})
+        SET(LIBS ${LIBS} ${CCFITS_LIBRARIES})
+        ADD_DEFINITIONS(-DHAVE_CCFITS)
+    ELSE()
+        MESSAGE(FATAL_ERROR "CCfits not found!")
+    ENDIF()
 ENDIF()
 
 SET(LIBS ${LIBS} ${OPENEXR_LIBRARIES})