SPI & file locations

I'm afraid this might sound rather dumb.  I'm hoping I can just get a
little clarification about file locations.

I've just started playing w/ SPI.  As a first stab, I thought I'd
compile a couple of the test applications in /contrib.

I pointed gcc to include files from /usr/local/pgsql - i.e. 'gcc ...
-I/usr/local/pgsql/include ...'.  This of course didn't work.
/usr/local/pgsql/include/executor/spi.h attempts to include files which
don't exist in the install directory.  They only exist in
/usr/local/src/postgresql-7.0/src/include (or wherever you put the
source).

After installation, shouldn't everything you need be in
/usr/local/pgsql?

It's simple enough to just use
/usr/local/src/postgresql-7.0/src/include.  But I don't know when to use
one, and when to use the other.

Sorry if this is a completely naive question.  I'm pretty much flying
solo here.  I'm an architect who's gotten frustrated with the
scaleability limitations of using something like MS Access.  I'm the
only person I know who uses any *NIX whatsoever, nevermind PostgreSQL.
C/C++ doesn't bother me, but I'm really not too familiar w/ *NIX file
conventions, etc.

-Ron-