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>