UseJava.cmake: simplify path logic

Move logic to determine relative path to .java file to after the file
extension check, so that it only executes when it is actually used.

Modules/UseJava.cmake

@@ -264,18 +264,18 @@ function(add_jar _TARGET_NAME)
         get_filename_component(_JAVA_PATH ${_JAVA_SOURCE_FILE} PATH)
         get_filename_component(_JAVA_FULL ${_JAVA_SOURCE_FILE} ABSOLUTE)
 
-        file(RELATIVE_PATH _JAVA_REL_BINARY_PATH ${CMAKE_JAVA_TARGET_OUTPUT_DIR} ${_JAVA_FULL})
-        file(RELATIVE_PATH _JAVA_REL_SOURCE_PATH ${CMAKE_CURRENT_SOURCE_DIR} ${_JAVA_FULL})
-        string(LENGTH ${_JAVA_REL_BINARY_PATH} _BIN_LEN)
-        string(LENGTH ${_JAVA_REL_SOURCE_PATH} _SRC_LEN)
-        if (${_BIN_LEN} LESS ${_SRC_LEN})
-            set(_JAVA_REL_PATH ${_JAVA_REL_BINARY_PATH})
-        else ()
-            set(_JAVA_REL_PATH ${_JAVA_REL_SOURCE_PATH})
-        endif ()
-        get_filename_component(_JAVA_REL_PATH ${_JAVA_REL_PATH} PATH)
-
         if (_JAVA_EXT MATCHES ".java")
+            file(RELATIVE_PATH _JAVA_REL_BINARY_PATH ${CMAKE_JAVA_TARGET_OUTPUT_DIR} ${_JAVA_FULL})
+            file(RELATIVE_PATH _JAVA_REL_SOURCE_PATH ${CMAKE_CURRENT_SOURCE_DIR} ${_JAVA_FULL})
+            string(LENGTH ${_JAVA_REL_BINARY_PATH} _BIN_LEN)
+            string(LENGTH ${_JAVA_REL_SOURCE_PATH} _SRC_LEN)
+            if (${_BIN_LEN} LESS ${_SRC_LEN})
+                set(_JAVA_REL_PATH ${_JAVA_REL_BINARY_PATH})
+            else ()
+                set(_JAVA_REL_PATH ${_JAVA_REL_SOURCE_PATH})
+            endif ()
+            get_filename_component(_JAVA_REL_PATH ${_JAVA_REL_PATH} PATH)
+
             list(APPEND _JAVA_COMPILE_FILES ${_JAVA_SOURCE_FILE})
             set(_JAVA_CLASS_FILE "${CMAKE_JAVA_CLASS_OUTPUT_PATH}/${_JAVA_REL_PATH}/${_JAVA_FILE}.class")
             set(_JAVA_CLASS_FILES ${_JAVA_CLASS_FILES} ${_JAVA_CLASS_FILE})