1740 Issue - Enabling and Disabling importers

-ASSIMP_BUILD_ALL_IMPORTERS_BY_DEFAULT will be respected by the macro
--using local variable ASSIMP_IMPORTER_ENABLED to include or exclude
--an importer
-Fixes individual importer ex: set(ASSIMP_BUILD_FBX_IMPORTER OFF)

-ASSIMP_BUILD_ALL_IMPORTERS_BY_DEFAULT is cached
--if assimp is a child cmake project, need to force cache
--from parent project
--ex: set(ASSIMP_BUILD_ALL_IMPORTERS_BY_DEFAULT CACHE INTERNAL FALSE)

code/CMakeLists.txt

@@ -208,8 +208,15 @@ OPTION(ASSIMP_BUILD_ALL_IMPORTERS_BY_DEFAULT "default value of all ASSIMP_BUILD_
 # macro to add the CMake Option ADD_ASSIMP_IMPORTER_<name> which enables compile of loader
 # this way selective loaders can be compiled (reduces filesize + compile time)
 MACRO(ADD_ASSIMP_IMPORTER name)
-  OPTION(ASSIMP_BUILD_${name}_IMPORTER "build the ${name} importer" ${ASSIMP_BUILD_ALL_IMPORTERS_BY_DEFAULT})
-  IF(ASSIMP_BUILD_${name}_IMPORTER)
+  IF (ASSIMP_BUILD_ALL_IMPORTERS_BY_DEFAULT)
+    set(ASSIMP_IMPORTER_ENABLED TRUE)
+    IF (DEFINED ASSIMP_BUILD_${name}_IMPORTER AND NOT ASSIMP_BUILD_${name}_IMPORTER)
+      set(ASSIMP_IMPORTER_ENABLED FALSE)
+    ENDIF ()
+  ELSE ()
+    set(ASSIMP_IMPORTER_ENABLED ${ASSIMP_BUILD_${name}_IMPORTER})
+  ENDIF ()
+  IF (ASSIMP_IMPORTER_ENABLED)
     LIST(APPEND ASSIMP_LOADER_SRCS ${ARGN})
     SET(ASSIMP_IMPORTERS_ENABLED "${ASSIMP_IMPORTERS_ENABLED} ${name}")
     SET(${name}_SRCS ${ARGN})