uwsgi-cgi: fix libcap detection

uwsgi-cgi's build system requires CPP to be set to avoid using include
path from the build system.  Otherwise it may wrongly detect
sys/capability.h of the host system and enables libcap support

CPP variable was once introduced into build system in 2017 but then
reverted in b957e45 ("rukes.mk: this patch broken grub2 builds")

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

net/uwsgi-cgi/Makefile

@@ -23,7 +23,7 @@ define Package/uwsgi-cgi
   SUBMENU:=Web Servers/Proxies
   TITLE:=The uWSGI server
   URL:=http://unbit.com/
-  DEPENDS:=+libopenssl +libpcre +jansson +libuuid +libxml2
+  DEPENDS:=+libcap +libopenssl +libpcre +jansson +libuuid +libxml2
 endef
 
 define Package/uwsgi-cgi-luci-support
@@ -39,6 +39,9 @@ define Package/uwsgi-cgi/description
 	The uWSGI project build with cgi profile
 endef
 
+MAKE_VARS+=\
+	CPP=$(TARGET_CROSS)cpp
+
 define Build/Compile
 	$(call Build/Compile/Default,PROFILE=cgi)
 endef
@@ -57,4 +60,4 @@ define Package/uwsgi-cgi-luci-support/install
 endef
 
 $(eval $(call BuildPackage,uwsgi-cgi))
-$(eval $(call BuildPackage,uwsgi-cgi-luci-support))
+$(eval $(call BuildPackage,uwsgi-cgi-luci-support))