FindQt3: fix warning when Qt3 is not found

-- Could NOT find Qt3 (missing:  QT_QT_LIBRARY QT_INCLUDE_DIR QT_MOC_EXECUTABLE)
CreateProcess failed QT_MOC_EXECUTABLE-NOTFOUND -v
CreateProcess failed QT_UIC_EXECUTABLE-NOTFOUND -version

Modules/FindQt3.cmake

@@ -296,8 +296,16 @@ IF(QT_FOUND)
   ENDIF(QT_QT_LIBRARY MATCHES "qt-mt")
 ENDIF(QT_FOUND)
 
-EXEC_PROGRAM(${QT_MOC_EXECUTABLE} ARGS "-v" OUTPUT_VARIABLE QTVERSION_MOC)
-EXEC_PROGRAM(${QT_UIC_EXECUTABLE} ARGS "-version" OUTPUT_VARIABLE QTVERSION_UIC)
+IF(QT_MOC_EXECUTABLE)
+  EXECUTE_PROCESS(COMMAND ${QT_MOC_EXECUTABLE} "-v"
+                  OUTPUT_VARIABLE QTVERSION_MOC
+                  ERROR_QUIET)
+ENDIF(QT_MOC_EXECUTABLE)
+IF(QT_UIC_EXECUTABLE)
+  EXECUTE_PROCESS(COMMAND ${QT_UIC_EXECUTABLE} "-version"
+                  OUTPUT_VARIABLE QTVERSION_UIC
+                  ERROR_QUIET)
+ENDIF(QT_UIC_EXECUTABLE)
 
 SET(_QT_UIC_VERSION_3 FALSE)
 IF("${QTVERSION_UIC}" MATCHES ".* 3..*")