Fix some cmake build system issues (#6147)

* [cmake] Switch to find_package for TIFF
- Included in cmake 3.5 and later
- Provides better configuration messages
* Add missing TIFF library to rtgui and rtexif
* Removed unnecessary quotes so that cmake sees the list as a list
* Add OpenMP to rtengine if OpenMP is enabled

rtgui/CMakeLists.txt

@@ -192,6 +192,7 @@ if(WIN32)
         ${GTK_INCLUDE_DIRS}
         ${LENSFUN_INCLUDE_DIRS}
         ${RSVG_INCLUDE_DIRS}
+        ${TIFF_INCLUDE_DIRS}
         )
     link_directories(. "${PROJECT_SOURCE_DIR}/rtexif"
         ${EXTRA_LIBDIR}
@@ -203,6 +204,7 @@ if(WIN32)
         ${GTK_LIBRARY_DIRS}
         ${LENSFUN_LIBRARY_DIRS}
         ${RSVG_LIBRARY_DIRS}
+        ${TIFF_LIBRARY_DIRS}
         )
 else()
     include_directories(${EXTRA_INCDIR}
@@ -221,6 +223,7 @@ else()
         ${LCMS_INCLUDE_DIRS}
         ${LENSFUN_INCLUDE_DIRS}
         ${RSVG_INCLUDE_DIRS}
+        ${TIFF_INCLUDE_DIRS}
         )
     link_directories(${EXTRA_LIBDIR}
         ${CANBERRA-GTK_LIBRARY_DIRS}
@@ -238,6 +241,7 @@ else()
         ${LCMS_LIBRARY_DIRS}
         ${LENSFUN_LIBRARY_DIRS}
         ${RSVG_LIBRARY_DIRS}
+        ${TIFF_LIBRARY_DIRS}
         )
 endif()