What do you mean by 'a macro with this value is parsed'? How was the BUILD_URL macro initialized? Literal % characters in the macro value should be escaped when creating the macro. The vbld_EscapeString system script function can be used for this.
http://www.kinook.com/VisBuildPro/Ma...scriptmisc.htm
To assign a macro value to another macro value, just copy the raw value without expanding. See the attached sample.