st/egl: Fix build on FreeBSD.
There is no libdl on FreeBSD. Based on patch from Thinker
<thinker@branda.to>, which is against 7.8.
This fixes fdo bug #29093.
diff --git a/configure.ac b/configure.ac
index 1214100..9619597 100644
--- a/configure.ac
+++ b/configure.ac
@@ -410,6 +410,7 @@
dnl has it in libc), or if libdl is needed to get it.
AC_CHECK_FUNC([dlopen], [],
[AC_CHECK_LIB([dl], [dlopen], [DLOPEN_LIBS="-ldl"])])
+AC_SUBST([DLOPEN_LIBS])
dnl See if posix_memalign is available
AC_CHECK_FUNC([posix_memalign], [DEFINES="$DEFINES -DHAVE_POSIX_MEMALIGN"])