problem with ./configure --with-gc=boehm

HI,
I'm trying to compile mono on mac os 10.5 using the boehm garbage collector. I obtain the following
error when typing ./configure --with-gc=boehm

...
checking gc.h usability... no
checking gc.h presence... no
checking for gc.h... no
checking gc/gc.h usability... no
checking gc/gc.h presence... no
checking for gc/gc.h... no
checking for GC_malloc in -lgc... no
configure: error: "GC requested but libgc not found! Install libgc or run configure with --with-gc=none."

For what I understood, libgc was included in mono, so why is requested me to install it? if I have to install it, which is the correct version for MacOS ?

Thank you,
Matteo