Switch from the non-standard upstream CMake Lua module to the one bundled
with CMake, use only the Lua version requested by the ebuild, and do not
semi-quietly ignore the absence of Lua.

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -433,7 +433,7 @@
 ENDIF(WANT_FONTCONFIG)
 
 IF(NOT PODOFO_BUILD_LIB_ONLY)
-FIND_PACKAGE(LUA)
+FIND_PACKAGE(Lua ${LUA_VERSION} EXACT REQUIRED)
 IF(LUA_FOUND)
 	# If we have lua, we can build podofoimpose.
 	MESSAGE("Lua found - PoDoFoImpose and PoDoFoColor will be built with Lua support")