beremiz

file isExecutable
add $(Beremiz installation)/mingw/bin in PATH on Windows platform

This fixes issue if multiple gcc installation are found in user's
PATH. Now gcc from Beremiz is used by default.
<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=""/>