Add b_lundef=false Meson option on FreeBSD
Add b_lundef=false Meson option on FreeBSD

The Meson option "b_lundef" need to be set to false on FreeBSD, because
the symbol "environ" is in crt1.o, which is not linked with shared
libraries. With Meson >=0.48.0 it is possible to set this option only
for FreeBSD. This patch changes meson.build to do that.

ascent12/wlroots,swaywm/wlroots,SirCmpwn/wlroots,ascent12/wlroots,swaywm/wlroots