Put quotes around ${MACRO_DEFINE_NAME} in if-statement (#516)

This is a safer way to check for an empty var I have found in the past.
Something strange happens with macro arguments (at least in older versions of
CMake).

This is in response to feedback from @KyleFromKitware while reviewing PR #521.

tribits/core/package_arch/TribitsAddOptionAndDefine.cmake

@@ -76,15 +76,15 @@ macro(tribits_add_option_and_define  USER_OPTION_NAME  MACRO_DEFINE_NAME
   )
   #message("TRIBITS_ADD_OPTION_AND_DEFINE: '${USER_OPTION_NAME}' '${MACRO_DEFINE_NAME}' '${DEFAULT_VALUE}'")
   set( ${USER_OPTION_NAME} "${DEFAULT_VALUE}" CACHE BOOL "${DOCSTRING}" )
-  if(NOT ${MACRO_DEFINE_NAME} STREQUAL "")
+  if(NOT "${MACRO_DEFINE_NAME}" STREQUAL "")
     if(${USER_OPTION_NAME})
       global_set(${MACRO_DEFINE_NAME} ON)
     else()
       global_set(${MACRO_DEFINE_NAME} OFF)
     endif()
   endif()
   tribits_pkg_export_cache_var(${USER_OPTION_NAME})
-  if(NOT ${MACRO_DEFINE_NAME} STREQUAL "")
+  if(NOT "${MACRO_DEFINE_NAME}" STREQUAL "")
     tribits_pkg_export_cache_var(${MACRO_DEFINE_NAME})
   endif()
 endmacro()