No include path in which to find <header file>

[Victor Havard]

I have successfully compiled an app and several days later tried to
compile it again and got the message
    No include path in which to find PalmOS.h

I've seen this before but have absolutely no idea what causes it.  The
compiler will never find any includes when this happens - not even if I
specify the fully qualified path name of the file in the #include.  I've
tried using -I flags to the compiler and renaming the files, etc. with
no luck.

Can anyone tell me what causes this error and how to fix it.  Right now
I can't compile anything. :-(

-- Victor