build: use pkgconfig to detect zlib

build: use pkgconfig to detect zlib

build: explicitly call PKG_PROG_PKG_CONFIG

Per the manual page, PKG_PROG_PKG_CONFIG needs to be invoked
explicitly if PKG_CHECK_MODULES might not happen (it is indeed stowed
in an AS_IF in kmod). Without this, funny failures can occur.
(As it did.)