Linking a shared library against a C function

In the course of developing my XML parser hooks I've been using an
external XML parser (expat) which is built as a shared library. The 
C functions I'm writing need to access functions within that library.

Is it OK just to link the .so of my backend function against the expat
library? i.e. to do

gcc -shared -lexpat -o pgxml.so pgxml.o 

as the link stage (it seems to work fine) -or is there a portability 
problem with this?

IF this is OK, would it be sensible to change the platform-specific 
makefile %.so rule to allow the specification of extra instance specific 
flags i.e. (example from Makefile.linux)

%.so: %.o       $(CC) -shared -o $@ $<

changed to:

%.so: %.o$(CC) -shared $(DLLINKFLAGS) -o $@ $< 

or something similar, which would prevent me from having to override
the global rule and allow greater portability.

Thanks

John