Make the disable for ENABLE_CMS be none

as it is for ENABLE_LIBOPENJPEG and for ENABLE_DCTDECODER
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5d141ad..0d753ef 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -51,7 +51,7 @@
 option(ENABLE_QT4 "Compile poppler qt4 wrapper." ON)
 option(ENABLE_QT5 "Compile poppler qt5 wrapper." ON)
 set(ENABLE_LIBOPENJPEG "openjpeg2" CACHE STRING "Use libopenjpeg for JPX streams. Possible values: openjpeg2, unmaintained, none. 'unmaintained' gives you the internal unmaintained decoder. Use at your own risk. 'none' compiles no JPX decoder at all. Default: openjpeg2")
-set(ENABLE_CMS "lcms2" CACHE STRING "Use color management system. Possible values: lcms2. Unset to disable color management system.")
+set(ENABLE_CMS "lcms2" CACHE STRING "Use color management system. Possible values: lcms2, none. 'none' disables color management system.")
 set(ENABLE_DCTDECODER "libjpeg" CACHE STRING "Use libjpeg for DCT streams. Possible values: libjpeg, unmaintained, none. will use libjpeg if available or fail if not. 'unmaintained' gives you the internal unmaintained decoder. Use at your own risk. 'none' compiles no DCT decoder at all. Default: libjpeg")
 option(ENABLE_LIBCURL "Build libcurl based HTTP support." ON)
 option(ENABLE_ZLIB "Build with zlib." ON)
@@ -222,8 +222,8 @@
 if(ENABLE_CMS STREQUAL "lcms2")
   find_package(LCMS2)
   set(USE_CMS ${LCMS2_FOUND})
-elseif(NOT ENABLE_CMS STREQUAL "")
-  MESSAGE(FATAL_ERROR "ENABLE_CMS is not lcms2 nor empty")
+elseif(NOT ENABLE_CMS STREQUAL "none")
+  message(FATAL_ERROR "Invalid ENABLE_CMS value: ${ENABLE_CMS}")
 endif()
 if(ENABLE_LIBCURL)
   find_package(CURL)