The options switch "Enable Mesa 3D (for GL modes)" is broken.
In fact it's broken in a very interesting way.  :-)

It turns out that these lines are the culprit:

    OPTIONS=MESAGL   "Enable Mesa 3D (for GL modes)"   off

    .if ${ARCH} == amd64
    WITH_MESAGL=    no
    .else
    WITH_MESAGL?=   yes
    .endif
    .if ${WITH_MESAGL} == yes
    CONFIGURE_ARGS+=        --with-mesa

First case:  options MESAGL is *disabled*.  Then the options
framework sets the variable WITHOUT_MESAGL=true.  The above
code (running on non-amd64) then sets WITH_MESAGL=yes anyway
enabling Mesa support even though the user didn't want it.

Second case:  options MESAGL is *enabled*.  As a result, the
variable WITH_MESAGL is set to "true" (not "yes"!).  The
above code only checks for the value "yes", therefore *not*
enabling Mesa support.

How-To-Repeat: 
On a FreeBSD/i386 machine:

cd /usr/ports/x11/xlockmore
make config
make

Notice that Mesa/GL modes are *not* included if you enabled
them in the config menu.  Notice that they *are* included
if you disabled them.