security/openvpn-auth-ldap: try to fix head fallout from -export-dynamic

The option is a linker option but is passed to cc verbatim, and this
causes fallout complaints on head i386 (13):

• lemon ---

cc -o lemon lemon.o -export-dynamic
ld: warning: cannot find entry symbol xport-dynamic; defaulting to 0x4049B0

> making all in src

• all ---
• auth-ldap.o ---

cc -fPIC -O2 -pipe -fPIC -fstack-protector-strong -DLDAP_DEPRECATED -fno-strict-aliasing -Wno-import -L/usr/local/lib -I/usr/local/include -fPIC -I/usr/local/include -D_THREAD_SAFE -fobjc-exceptions -fno-strict-aliasing -O2 -pipe -fPIC -fstack-protector-strong -DLDAP_DEPRECATED -fno-strict-aliasing -DHAVE_CONFIG_H -Wall -I/usr/local/include -I/usr/local/include -I/usr/local/include -I/usr/local/include -c auth-ldap.m -o auth-ldap.o -I. -I../src -I.. -I../src -I. -I../tests -I../tests

• TRConfigParser.m ---

./tools/lemon -T../tools/lempar.c -m -q TRConfigParser.lemon -OTRConfigParser.m
Segmentation fault (core dumped)

• [TRConfigParser.m] Error code 139

NOTE! With devel/check exactly at 0.15.1, this fails to build due to a regression
in check 0.15.1. 0.15.0 is fine, and the bug is reported here:

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=248274 (FreeBSD)
https://github.com/libcheck/check/issues/293 (upstream)

http://beefy17.nyi.freebsd.org/data/head-i386-default/p543393_s363499/logs/openvpn-auth-ldap-2.0.4.log

This is more fallout to permit narrowing down failures:

>> Building security/openvpn-auth-ldap

build started at Sun Jul 26 01:01:36 UTC 2020
port directory: /usr/ports/security/openvpn-auth-ldap
package name: openvpn-auth-ldap-2.0.4
building for: FreeBSD head-i386-default-job-12 13.0-CURRENT FreeBSD 13.0-CURRENT 1300101 i386
maintained by: mandree@FreeBSD.org
Makefile ident: $FreeBSD: head/security/openvpn-auth-ldap/Makefile 527679 2020-03-03 15:11:46Z mat $
Poudriere version: 3.2.8-5-gc81843e5
Host OSVERSION: 1300100
Jail OSVERSION: 1300101
Job Id: 12

! Jail is newer than host. (Jail: 1300101, Host: 1300100) !
! This is not supported. !
! Host kernel must be same or newer than jail. !
! Expect build failures. !

---Begin Environment---
SHELL=/bin/csh
UNAME_p=i386
UNAME_m=i386
OSVERSION=1300101
UNAME_v=FreeBSD 13.0-CURRENT 1300101
UNAME_r=13.0-CURRENT
BLOCKSIZE=K
MAIL=/var/mail/root
STATUS=1
HOME=/root
PATH=/sbin:/bin:/usr/sbin:/usr/bin:/usr/local/sbin:/usr/local/bin:/root/bin
LOCALBASE=/usr/local
USER=root
LIBEXECPREFIX=/usr/local/libexec/poudriere
POUDRIERE_VERSION=3.2.8-5-gc81843e5
MASTERMNT=/usr/local/poudriere/data/.m/head-i386-default/ref
POUDRIERE_BUILD_TYPE=bulk
PACKAGE_BUILDING=yes
SAVED_TERM=
PWD=/usr/local/poudriere/data/.m/head-i386-default/ref/.p/pool
P_PORTS_FEATURES=FLAVORS SELECTED_OPTIONS
MASTERNAME=head-i386-default
SCRIPTPREFIX=/usr/local/share/poudriere
OLDPWD=/usr/local/poudriere/data/.m/head-i386-default/ref/.p
SCRIPTPATH=/usr/local/share/poudriere/bulk.sh
POUDRIEREPATH=/usr/local/bin/poudriere
---End Environment---