Allow packages to add source code to the project

By returning a ${package}_SOURCES variable with the names of the code,
it will be propagated to ${project}_SOURCES and subsequently compiled
into the library.

AGMG (Algebraic Multigrid) uses this functionality.

cmake/Modules/OpmFind.cmake

@@ -36,6 +36,7 @@ include (Duplicates)
 
 # list of suffixes for all the project variables
 set (_opm_proj_vars
+  SOURCES
   LINKER_FLAGS
   LIBRARIES
   DEFINITIONS