opensmtpd: force vendored arc4random with correct prototype.

Previously, arc4random was called with implicit prototype, which
resulted in bad uids like ffffffff85743157 half of the time...

srcpkgs/opensmtpd/template

@@ -1,11 +1,15 @@
 # Template file for 'opensmtpd'
 pkgname=opensmtpd
 version=6.0.2p1
-revision=3
+revision=4
 build_style=gnu-configure
 configure_args="--sysconfdir=/etc/smtpd --sbindir=/usr/bin
  --with-path-socket=/run --with-mantype=doc --with-pie --with-table-db
  --with-path-CAfile=/etc/ssl/certs/ca-certificates.crt
+ ac_cv_search_arc4random=no
+ ac_cv_func_arc4random=no
+ ac_cv_func_arc4random_buf=no
+ ac_cv_func_arc4random_uniform=no
  ac_cv_func_explicit_bzero=no
  ac_cv_func_reallocarray=no
  ac_cv_func_strlcat=no
@@ -23,6 +27,8 @@ homepage="https://www.opensmtpd.org"
 distfiles="https://www.opensmtpd.org/archives/${pkgname}-${version}.tar.gz"
 checksum=2af9b6d08784c7e546bf124bb61e311a6aa0c9835507710a76f5c242383190ac
 
+CFLAGS=-D_DEFAULT_SOURCE
+
 system_accounts="_smtpd _smtpq"
 
 case "$XBPS_TARGET_MACHINE" in