ltrace: disable libunwind for uClibc toolchains

ltrace requires that libunwind is built with backtrace() support.
For the internal uClibc toolchain we don't enable it, and for external
uClibc toolchains we can't know.
It's also unavailable for static uClibc toolchains.
So just disable libunwind support for uClibc toolchains in general.
Fixes:
http://autobuild.buildroot.net/results/ee0/ee037a19590fb85c64f97f78f74bcfd4d7766706/

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/ltrace/ltrace.mk

@@ -13,4 +13,15 @@ LTRACE_CONF_OPT = --disable-werror
 LTRACE_LICENSE = GPLv2
 LTRACE_LICENSE_FILES = COPYING
 
+# ltrace can use libunwind only if libc has backtrace() support
+# We don't normally do so for uClibc and we can't know if it's external
+ifeq ($(BR2_PACKAGE_LIBUNWIND),y)
+ifeq ($(BR2_TOOLCHAIN_USES_UCLIBC),)
+LTRACE_CONF_OPT += --with-libunwind=yes
+LTRACE_DEPENDENCIES += libunwind
+else
+LTRACE_CONF_OPT += --with-libunwind=no
+endif
+endif
+
 $(eval $(autotools-package))