How to find out about system paths?

["Stephen C. Biggs" <s_c_biggs at bigfoot dot com>]

I have seen this question addressed in the Usenet group a few times 
with not a reply.  I'll try here.

I see all over the documentation about "default system paths" that 
gcc uses and that you can modify them with -isystem, and a myriad 
of other options.

My question is: how do you see what your local version has already 
defined... AND how hard is it to define it system-wide?  I would like 
to be able to have /usr/local/include come before /usr/include and 
the "fixedheader" directory... also /usr/local/lib before /lib,

something along the lines of:
$something_include$=/usr/local/include:/usr/include

etc... since I would rather not have to pass this through every 
makefile...

Please forgive the newbie question.