Define uppercase variant when testing for existence

CMake will do some uppercasing of its own (defining OPM-PARSER_FOUND)
so it works best if we specify the correct uppercase version and sets
the lowercase afterwards.

cmake/Modules/Findopm-parser.cmake

@@ -118,7 +118,8 @@ endif ()
 
 # if the test program didn't compile, but was required to do so, bail
 # out now and display an error; otherwise limp on
-find_package_handle_standard_args (opm-parser
+find_package_handle_standard_args (OPM_PARSER
   DEFAULT_MSG
   OPM_PARSER_INCLUDE_DIR OPM_PARSER_LIBRARIES HAVE_OPM_PARSER
   )
+set (opm-parser_FOUND ${OPM_PARSER_FOUND})