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.

meson.build

@@ -111,6 +111,12 @@ wlr_deps += [
 	math,
 ]
 
+if host_machine.system().startswith('freebsd')
+	override_options = ['b_lundef=false']
+else
+	override_options = []
+endif
+
 symbols_file = 'wlroots.syms'
 symbols_flag = '-Wl,--version-script,@0@/@1@'.format(meson.current_source_dir(), symbols_file)
 lib_wlr = library(
@@ -122,6 +128,7 @@ lib_wlr = library(
 	install: true,
 	link_args : symbols_flag,
 	link_depends: symbols_file,
+	override_options: override_options,
 )
 
 wlroots = declare_dependency(