Automake and files with the same name

I've a C++ autoconf managed project that I'm adapting to compile on FreeBSD hosts.
The original system was Linux so I made one AM_CONDITIONAL to distinguish the host I'm building and separate the code into system specific files.

configure.ac

AC_CANONICAL_HOST
AM_CONDITIONAL([IS_FREEBSD],false)
case $host in
        *free*)    
            AC_DEFINE([IS_FREEBSD],[1],[FreeBSD Host])
            AM_CONDITIONAL([IS_FREEBSD],true)
            BP_ADD_LDFLAG([-L/usr/local/lib])
                ;;
esac

Makefile.am

lib_LTLIBRARIES=mylib.la
mylib_la_SOURCES=a.cpp \
                 b.cpp

if IS_FREEBSD
    mylib_la_SOURCES+=freebsd/c.cpp
else
    mylib_la_SOURCES+=linux/c.cpp
endif

When I run automake it fails with this kind of message:

Makefile.am: object `c.lo' created by `linux/c.cpp' and `freebsd/c.cpp'

Any ideas on how to configure automake to respect this conditional even in the Makefile.in build proccess?

I this works if the files have diferent names, but it's c++ code and I'm trying to keep the filenames the same as the class name.

Thanks in advance!