The ./configure logic in xmlsec-1.2.18 on OS X 10.6 determines that XMLSEC_DL_LIBLTDL should be set, but it includes a stray comma in the value. For example, I see the following as the first compiler command during 'make':

/bin/sh ../libtool --tag=CC   --mode=compile gcc -DHAVE_CONFIG_H -I. -I.. -DPACKAGE=\"xmlsec1\" -I../include -I../include -D__XMLSEC_FUNCTION__=__FUNCTION__ -DXMLSEC_NO_GOST=1 -DXMLSEC_DL_LIBLTDL=1,  -I/sw/include -I/sw/include/libxml2 -I/sw/include/libxml2 -I/sw/include   -I/sw/include  -g -O2 -MT app.lo -MD -MP -MF .deps/app.Tpo -c -o app.lo app.c

Note the "-DXMLSEC_DL_LIBLTDL=1," in there.