lang/scm: try unbreaking on the architectures without sbrk(2)

Upstream's code seems to have special-handling for such platforms.

lang/scm/Makefile

@@ -13,14 +13,11 @@ WWW=		https://people.csail.mit.edu/jaffer/SCM
 LICENSE=	LGPL3+
 LICENSE_FILES=	${WRKSRC}/COPYING.LESSER
 
-BROKEN_aarch64=		fails to link: missing sbrk
-BROKEN_riscv64=		fails to link: missing sbrk
+LIB_DEPENDS=	libgnuregex.so:devel/libgnuregex
 
-LIB_DEPENDS=	libgnuregex.so.[5-9]:devel/libgnuregex
+USES=		readline zip
 
 WRKSRC=		${WRKDIR}/${PORTNAME}
-
-USES=		readline zip
 HAS_CONFIGURE=	yes
 CONFIGURE_ENV=	--prefix=${PREFIX}
 MAKE_JOBS_UNSAFE=	yes
@@ -61,6 +58,10 @@ SCM_MODULES_X11=x.so
 USES+=		compiler:gcc-c++11-lib
 .endif
 
+.if ${ARCH} == aarch64 || ${ARCH} == riscv64
+CFLAGS+=	-DLACK_SBRK
+.endif
+
 .include <bsd.port.pre.mk>
 
 .if ${PORT_OPTIONS:MX11}