fix cross-build for cygwin

nix-meson-build-support/common/meson.build

@@ -5,6 +5,15 @@ if not (host_machine.system() == 'windows' and cxx.get_id() == 'gcc')
   deps_private += dependency('threads')
 endif
 
+if host_machine.system() == 'cygwin'
+  # -std=gnu on cygwin defines 'unix', which conflicts with the namespace
+  add_project_arguments(
+    '-D_POSIX_C_SOURCE=200809L',
+    '-D_GNU_SOURCE',
+    language : 'cpp',
+  )
+endif
+
 add_project_arguments(
   '-Wdeprecated-copy',
   '-Werror=suggest-override',

src/libexpr/meson.build

@@ -69,6 +69,10 @@ if bdw_gc.found()
     define_value = cxx.has_function(funcspec).to_int()
     configdata_priv.set(define_name, define_value)
   endforeach
+  if host_machine.system() == 'cygwin'
+    # undefined reference to `__wrap__Znwm'
+    configdata_pub.set('GC_NO_INLINE_STD_NEW', 1)
+  endif
 endif
 # Used in public header. Affects ABI!
 configdata_pub.set('NIX_USE_BOEHMGC', bdw_gc.found().to_int())