Handle REQUIRED in our CMake "find" modules

These are expected to abort if they fail to find the relevant software
and "REQUIRED" is specified.

cmake/Modules/FindFfmpeg.cmake

@@ -15,3 +15,8 @@ else()
 	find_library(SWSCALE_LIBRARIES NAMES swscale)
 	find_package_handle_standard_args(SWSCALE DEFAULT_MSG SWSCALE_LIBRARIES SWSCALE_INCLUDE_DIRS)
 endif()
+
+if(Ffmpeg_FIND_REQUIRED AND
+   (NOT AVCODEC_FOUND OR NOT AVUTIL_FOUND OR NOT SWSCALE_FOUND))
+	message(FATAL_ERROR "Could not find FFMPEG")
+endif()

cmake/Modules/FindGmp.cmake

@@ -11,3 +11,7 @@ if(NOT GMP_FOUND)
   find_library(GMP_LIBRARIES NAMES gmp)
   find_package_handle_standard_args(GMP DEFAULT_MSG GMP_LIBRARIES GMP_INCLUDE_DIRS)
 endif()
+
+if(Gmp_FIND_REQUIRED AND NOT GMP_FOUND)
+	message(FATAL_ERROR "Could not find GMP")
+endif()

cmake/Modules/FindNettle.cmake

@@ -15,3 +15,7 @@ endif()
 if (NOT HOGWEED_FOUND OR NOT GMP_FOUND)
   set(NETTLE_FOUND 0)
 endif()
+
+if(Nettle_FIND_REQUIRED AND NOT NETTLE_FOUND)
+	message(FATAL_ERROR "Could not find Nettle")
+endif()

cmake/Modules/FindPixman.cmake

@@ -7,3 +7,7 @@ else()
 	find_library(PIXMAN_LIBRARIES NAMES pixman-1)
 	find_package_handle_standard_args(PIXMAN DEFAULT_MSG PIXMAN_LIBRARIES PIXMAN_INCLUDE_DIRS)
 endif()
+
+if(Pixman_FIND_REQUIRED AND NOT PIXMAN_FOUND)
+	message(FATAL_ERROR "Could not find Pixman")
+endif()