PKG_CONFIG_PATH on Windows

[John Emmas <john creativepost co uk>]

For whatever reason, my only copy of pkg-config.exe seems to be in a 
folder called "C:\Program Files\gtkmm\bin".  I don't know if that's 
normally the case - but at least it explains why I'm asking this 
question here.  My problem is that if I attempt to run pkg-config on my 
system, it will only find ".pc" files if they happen to be in the 
following folder:-

        C:\Program Files\gtkmm\lib\pkgconfig

but on my system, my ".pc" files are almost all on the drive that I use 
for building (F:).  Yesterday, I discovered an environment variable 
called PKG_CONFIG_PATH.  I needed to experiment a bit - but eventually, 
the following command seemed to make pkg-config look in the right folder:-

        set PKG_CONFIG_PATH=F:\+GTK-SOURCES\gnu-windows\lib\pkgconfig;

But this morning it's simply stopped working again!  Issuing the above 
command does stop it from looking on my C: drive - but it doesn't find 
my ".pc" files any more, even though that path is correct (including the 
'+' sign).  I've tried with and without the semicolon on the end.  I've 
tried using forward slashes instead of back slashes and I've also tried 
enclosing the path in quote marks - but nothing seems to work.  Maybe 
something more is needed, apart from just setting that env var.  Is 
there another step that I need to know about??

John