emulators/hercules4sdl: unbreak the build on Darwin.

No revbump needed since the change is limited to Darwin, where no built
package previously existed (at least for Darwin 20.6.0 which I tested).

emulators/hercules4sdl/Makefile

@@ -1,4 +1,4 @@
-# $NetBSD: Makefile,v 1.1 2021/11/06 16:43:23 rhialto Exp $
+# $NetBSD: Makefile,v 1.2 2021/11/13 12:44:39 rhialto Exp $
 
 PKGNAME=	hercules4sdl-4.3
 DISTNAME=	Release_${PKGVERSION_NOREV}
@@ -33,11 +33,17 @@ SUBST_FILES.prefix=	hercules.cnf
 SUBST_SED.prefix=	-e 's,/usr/local,${PREFIX},g'
 
 CONFIGURE_ARGS+=	--enable-extpkgs=${PREFIX}/lib/hercules4sdl
+.if ${OPSYS} == "Darwin"
+# There is a configure check that uses C++ to check if the compiler is
+# GNU, and on Darwin it thinks GNU C++ is needed to create shared
+# libraries, even though no actual C++ is used. Or something.
+CONFIGURE_ENV+=		ac_cv_cxx_compiler_gnu=yes
+.endif
 
 INSTALLATION_DIRS+=	share/examples/hercules
 
 pre-configure:
-	${RM} ${WRKSRC}/ltdl.h ${WRKSRC}/ltdl.c
+	${RM} -f ${WRKSRC}/ltdl.h ${WRKSRC}/ltdl.c
 	cd ${WRKSRC} && ./autogen.sh
 
 post-install: