Add _C11_SOURCE feature test macro on FreeBSD

This will restrict the default namespace set on FreeBSD to the C11
standard (everything is visible by default), which will prevent possible
conflicts with symbols hidden behing __BSD_VISIBLE.

meson.build

@@ -22,6 +22,10 @@ datadir = get_option('datadir')
 sysconfdir = get_option('sysconfdir')
 prefix = get_option('prefix')
 
+if is_freebsd
+	add_project_arguments('-D_C11_SOURCE', language: 'c')
+endif
+
 swayidle_deps = []
 
 jsonc          = dependency('json-c', version: '>=0.13')