multimedia/gpac-libgpac: fix build with clang 18

Clang 18 no longer accepts -export-dynamic, which is a linker option,
resulting in an error building multimedia/gpac-libgpac:

  cc: error: unknown argument: '-export-dynamic'

Fix this by using the correct spelling, -Wl,--export-dynamic.

PR:		276143
MFH:		2024Q1

multimedia/gpac-libgpac/Makefile

@@ -1,7 +1,7 @@
 PORTNAME=	gpac
 DISTVERSIONPREFIX=	v
 DISTVERSION=	1.0.0
-PORTREVISION=	2
+PORTREVISION=	3
 PORTEPOCH=	1
 CATEGORIES=	multimedia
 PKGNAMESUFFIX=	-libgpac
@@ -56,7 +56,8 @@ PNG_CONFIGURE_OFF=	--use-png=no
 
 post-patch:
 	${REINPLACE_CMD} -e 's|s/\.\*\\(\[0-9\]\\)\\+\$$/\\1/|s/^[^0-9]*//| ;\
-		/CFLAGS=/s/-O3//' ${WRKSRC}/configure
+		/CFLAGS=/s/-O3// ; s/-export-dynamic/-Wl,--export-dynamic/' \
+		${WRKSRC}/configure
 
 post-install:
 	${INSTALL_DATA} ${WRKSRC}/gpac.pc ${STAGEDIR}${PREFIX}/libdata/pkgconfig