build: fix cflags passing for mingw miniupnpc

Cherry-pick of a patch upstreamed to miniupnpc, see here:
miniupnp/miniupnp#619.

depends/packages/miniupnpc.mk

@@ -3,7 +3,7 @@ $(package)_version=2.2.2
 $(package)_download_path=https://miniupnp.tuxfamily.org/files/
 $(package)_file_name=$(package)-$($(package)_version).tar.gz
 $(package)_sha256_hash=888fb0976ba61518276fe1eda988589c700a3f2a69d71089260d75562afd3687
-$(package)_patches=dont_leak_info.patch
+$(package)_patches=dont_leak_info.patch respect_mingw_cflags.patch
 
 define $(package)_set_vars
 $(package)_build_opts=CC="$($(package)_cc)"
@@ -13,7 +13,8 @@ $(package)_build_env+=CFLAGS="$($(package)_cflags) $($(package)_cppflags)" AR="$
 endef
 
 define $(package)_preprocess_cmds
-  patch -p1 < $($(package)_patch_dir)/dont_leak_info.patch
+  patch -p1 < $($(package)_patch_dir)/dont_leak_info.patch && \
+  patch -p1 < $($(package)_patch_dir)/respect_mingw_cflags.patch
 endef
 
 define $(package)_build_cmds

depends/patches/miniupnpc/respect_mingw_cflags.patch

@@ -0,0 +1,23 @@
+commit fec515a7ac9991a0ee91068fda046b54b191155e
+Author: fanquake <[email protected]>
+Date:   Wed Jul 27 15:52:37 2022 +0100
+
+    build: respect CFLAGS in makefile.mingw
+
+    Similar to the other Makefile.
+
+    Cherry-pick of https://github.com/miniupnp/miniupnp/pull/619.
+
+diff --git a/Makefile.mingw b/Makefile.mingw
+index 2bff7bd..88430d2 100644
+--- a/Makefile.mingw
++++ b/Makefile.mingw
+@@ -19,7 +19,7 @@ else
+ RM = rm -f
+ endif
+ #CFLAGS = -Wall -g -DDEBUG -D_WIN32_WINNT=0X501
+-CFLAGS = -Wall -W -Wstrict-prototypes -Os -DNDEBUG -D_WIN32_WINNT=0X501
++CFLAGS ?= -Wall -W -Wstrict-prototypes -Os -DNDEBUG -D_WIN32_WINNT=0X501
+ LDLIBS = -lws2_32 -liphlpapi
+ # -lwsock32
+ # -liphlpapi is needed for GetBestRoute() and GetIpAddrTable()