Build libunwind only with CONFIG_CXX=yes

libunwind contains C++ sources, so it's not possible to build
it without a C++ compiler.

platform/Makefile.inc

@@ -11,8 +11,10 @@ INSTALLTGTS=	librumpkern_bmktc_install
 ifneq (${KERNONLY},true)
 TARGETS+=	userlibs
 INSTALLTGTS+=	librumprun_base_install librumprun_tester_install2
+ifeq (${CONFIG_CXX},yes)
 INSTALLTGTS+=	libunwind_install2
 endif
+endif
 
 ifeq (${BUILDRR},true)
 CPPFLAGS+=      -I${RROBJ}/dest.stage/include
@@ -60,8 +62,11 @@ $(eval $(call BUILDLIB_target,libbmk_rumpuser))
 $(eval $(call BUILDLIB_target,librumpkern_bmktc))
 $(eval $(call BUILDLIB_target,librumprun_base))
 $(eval $(call BUILDLIB_target,librumprun_tester))
-$(eval $(call BUILDLIB_target,libunwind))
 $(eval $(call BUILDLIB_target,libcompiler_rt,RUMPSRC=${RUMPSRC}))
+ifeq (${CONFIG_CXX},yes)
+$(eval $(call BUILDLIB_target,libunwind))
+LIBUNWIND= ${RROBJLIB}/libunwind/libunwind.a
+endif
 
 PSEUDOSTUBS:= ${RROBJ}/pseudolinkstubs
 
@@ -72,7 +77,7 @@ ${RROBJ}/bmk.ldscript: ${LDSCRIPT}
 	ln -sf $< $@
 
 commonlibs: platformlibs userlibs
-userlibs: ${PSEUDOSTUBS}.o ${RROBJLIB}/librumprun_base/librumprun_base.a ${RROBJLIB}/librumprun_tester/librumprun_tester.a ${RROBJLIB}/libunwind/libunwind.a
+userlibs: ${PSEUDOSTUBS}.o ${RROBJLIB}/librumprun_base/librumprun_base.a ${RROBJLIB}/librumprun_tester/librumprun_tester.a ${LIBUNWIND}
 platformlibs: ${RROBJLIB}/libbmk_core/libbmk_core.a ${RROBJLIB}/libbmk_rumpuser/libbmk_rumpuser.a ${RROBJ}/bmk.ldscript
 rumpkernlibs: ${RROBJLIB}/librumpkern_bmktc/librumpkern_bmktc.a
 compiler_rt: ${RROBJLIB}/libcompiler_rt/libcompiler_rt.a