Relative Path in \InputIfFileExists

[Thomas Templin]

In Layout > Latex Preamble (german Menu: Layout > Latex Vorspann)
\InputIfFileExists{/home/user/.lyx/include/mymac.inc}
works fine but 
\InputIfFileExists{~/user/.lyx/include/mymac.inc}
does not.
So, is there any way to tell Lyx to use a relative path like 
$LYX_INCLUDE_DIR ?

I think there must be a way but up to now I don't figure it out.
Bye,
Thomas
-- 
Thomas Templin
Postfach 2519
26365 Wilhelmshaven