nomlib-config.cmake: Clean up handling of NOMLIB_DEPS_ROOT var

* Collapse separate handling of NOMLIB_DEPS_PREFIX and
ENV{NOMLIB_DEPS_PREFIX} to one internal var (NOMLIB_DEPS_ROOT)
by appending them at script init, instead of treating the two
vars separately. This helps prevent us from accidentally modifying
the input vars and potentially passing unexpected results back to the
end-user of this module.

* Add validation checks for path existence of the deps prefix before
continuing; this takes the burden off of the end-user of this module.

Resources/CMake/nomlib-config.cmake

@@ -67,6 +67,24 @@
 # To distribute this file outside of CMake, substitute the full license text
 # for the above reference.
 
+set( NOMLIB_DEPS_ROOT "NOTFOUND" )
+
+if( EXISTS ${NOMLIB_DEPS_PREFIX} )
+  # IMPORTANT: NOMLIB_DEPS_PREFIX will be an external CMake variable within
+  # this scope, of which **must** already be set by the parent script calling
+  # FIND_PACKAGE, and should **never** be modified for sake of the parent
+  # script.
+  list( APPEND NOMLIB_DEPS_ROOT ${NOMLIB_DEPS_PREFIX} )
+endif( EXISTS ${NOMLIB_DEPS_PREFIX} )
+
+if( DEFINED ENV{NOMLIB_DEPS_PREFIX} AND EXISTS ENV{NOMLIB_DEPS_PREFIX} )
+  list( APPEND NOMLIB_DEPS_ROOT ENV{NOMLIB_DEPS_PREFIX} )
+endif( DEFINED ENV{NOMLIB_DEPS_PREFIX} AND EXISTS ENV{NOMLIB_DEPS_PREFIX} )
+
+if( NOMLIB_DEPS_ROOT STREQUAL "NOTFOUND" )
+  message( FATAL_ERROR "An absolute directory path to the engine's dependencies must be set. See README.md for details." )
+endif( NOMLIB_DEPS_ROOT STREQUAL "NOTFOUND" )
+
 if( CMAKE_SYSTEM_NAME STREQUAL "Darwin" )
   set( NOM_PLATFORM_OSX TRUE )
 elseif( CMAKE_SYSTEM_NAME STREQUAL "Windows" )
@@ -180,7 +198,6 @@ macro( find_nom_dependency output_var output_name )
   find_library( ${output_var} NAMES ${ARGN}
                 PATHS
                 ${NOMLIB_DEPS_ROOT}
-                $ENV{NOMLIB_DEPS_ROOT}
                 ${NOM_SEARCH_PREFIX_PATHS}
                 PATH_SUFFIXES
                 ${NOM_LIBRARY_PATH_SUFFIXES} # osx, *nix
@@ -215,7 +232,6 @@ if( NOMLIB_CORE_FOUND )
   find_path(  SDL2_INCLUDE_DIR SDL.h
               PATHS
               ${NOMLIB_DEPS_ROOT}
-              $ENV{NOMLIB_DEPS_ROOT}
               ${NOM_SEARCH_PREFIX_PATHS}
               PATH_SUFFIXES SDL2/include include
   )
@@ -263,7 +279,6 @@ if( NOMLIB_GUI_FOUND )
   find_path(  LIBROCKET_INCLUDE_DIR Rocket/Core
               PATHS
               ${NOMLIB_DEPS_ROOT}
-              $ENV{NOMLIB_DEPS_ROOT}
               ${NOM_SEARCH_PREFIX_PATHS}
               PATH_SUFFIXES libRocket/include include
   )