fix problem with clearing non-empty location field after changing type of config variable
introduced by cd9db17 ("clear variable location cell in variable
panel, if variable class or type changed").
Closes #26
<xsd:attribute name="Compiler" type="xsd:string" use="optional" default="gcc"/>
<xsd:attribute name="CFLAGS" type="xsd:string" use="optional" default=""/>
<xsd:attribute name="Linker" type="xsd:string" use="optional" default="gcc"/>
<xsd:attribute name="LDFLAGS" type="xsd:string" use="optional" default=""/>