databases/postgis21 doesn't compile without the patch in PR-186563 (which works but hasn't been committed). But even after fixing that, postgis still doesn't work:

# sudo -u pgsql psql -d "my_db" -c "create extension postgis"
ERROR:  could not open extension control file "/usr/local/share/postgresql/extension/postgis.control": No such file or directory

And indeed, that file doesn't exist:

# find /usr/local -name postgis.control | wc -l
       0

Why? Because the pkg-plist for postgis21 lists half of its fundamental requirements as required by the RASTER option. An except of files that require RASTER that actually are required by *any* install of postgis:

%%RASTER%%share/postgresql/extension/postgis--2.1.0rc1--%%PORTVERSION%%.sql
%%RASTER%%share/postgresql/extension/postgis--2.1.0rc2--%%PORTVERSION%%.sql
%%RASTER%%share/postgresql/extension/postgis--2.1.0rc3--%%PORTVERSION%%.sql
%%RASTER%%share/postgresql/extension/postgis--unpackaged--%%PORTVERSION%%.sql
%%RASTER%%share/postgresql/extension/postgis.control

In the Makefile, %%RASTER%% is changed to comment out the plist line when RASTER isn't built:

.if ${PORT_OPTIONS:MRASTER}
.if ${OSVERSION} < 900033
BROKEN=                 Raster suppport is broken on FreeBSD < 9.x
.endif
LIB_DEPENDS+=           libgdal.so:${PORTSDIR}/graphics/gdal
CONFIGURE_ARGS+=        --with-raster --with-gdalconfig=${LOCALBASE}/bin/gdal-config
PLIST_SUB+=             RASTER=""
.else
CONFIGURE_ARGS+=        --without-raster
PLIST_SUB+=             RASTER="@comment "
.endif

RASTER is a very large requirement that pulls in gdal which pulls in image processing dependencies (which as you can imagine usually means pulling in X11). It's not a requirement of postgis in general

Fix: 

I imagine that the fix is to go through pkg-plist and remove %%RASTER%% where it doesn't make sense. I don't know postgis well enough to tell these apart myself
How-To-Repeat: Install databases/postgis21 (this may mean applying the long-outstanding patch from PR-186563 to resolve the compile error on iconv). Then try to use it with a command like:

# sudo -u pgsql psql -d "my_db" -c "create extension postgis;"