meson: set _GNU_SOURCE on cygwin

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
diff --git a/meson.build b/meson.build
index a7a51bc..7a7793b 100644
--- a/meson.build
+++ b/meson.build
@@ -481,7 +481,7 @@
 endif
 
 # TODO: this is very incomplete
-if host_machine.system() == 'linux'
+if ['linux', 'cygwin'].contains(host_machine.system())
   pre_args += '-D_GNU_SOURCE'
 endif