CFLAGS/LDFLAGS vs. --with

[Murray S. Kucherawy]

What is the general wisdom when deciding whether to add support for a new 
"-with" vs. just encouraging use of CFLAGS/LDFLAGS on the command line?

In particular I'm talking about openssl here.  I have a package that needs it.  
Several systems come stock with an older version in /usr and then people 
install a newer one in /usr/local or perhaps someplace weird like /usr/pkg.  Is 
it more common or correct to tell people to do:

./configure CFLAGS=-I/usr/pkg/include LDFLAGS=-L/usr/pkg/lib

...or add support for a "-with-openssl" that adds additional variables during 
compilation pointing to the one the user wants my package to use?

Unless I'm missing something (which is possible because I'm still learning 
autoconf), adding the "-with" will mean I can't use AC_CHECK_HEADERS to search 
for various openssl includes and will have to do my own tests for the required 
headers, libraries and functions.  Or is it possible to arrange that 
AC_CHECK_HEADERS applies its work to the additional path provided via the 
"-with"?  I didn't see such a capability described in the info pages, but I may 
not have looked in the right place.

-MSK