GenerateExportHeader: Fix wrong use of IS_ABSOLUTE (#12645)

Test content of _GEH_EXPORT_FILE_NAME, not variable name.

Signed-off-by: Michael Wild <[email protected]>

Modules/GenerateExportHeader.cmake

@@ -298,7 +298,7 @@ macro(_DO_GENERATE_EXPORT_HEADER TARGET_LIBRARY)
     set(EXPORT_MACRO_NAME ${_GEH_PREFIX_NAME}${_GEH_EXPORT_MACRO_NAME})
   endif()
   if(_GEH_EXPORT_FILE_NAME)
-    if(IS_ABSOLUTE _GEH_EXPORT_FILE_NAME)
+    if(IS_ABSOLUTE ${_GEH_EXPORT_FILE_NAME})
       set(EXPORT_FILE_NAME ${_GEH_EXPORT_FILE_NAME})
     else()
       set(EXPORT_FILE_NAME "${CMAKE_CURRENT_BINARY_DIR}/${_GEH_EXPORT_FILE_NAME}")