cairo: fix build when toolchain has no thread support

Fixes the build failure visible at
http://autobuild.buildroot.net/results/d49/d491de5a774592929a59f3bfd0b1fd8cbc089ac0

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/cairo/cairo.mk

@@ -36,6 +36,10 @@ CAIRO_CONF_ENV = ac_cv_func_posix_getpwuid_r=yes glib_cv_stack_grows=no \
 		ac_cv_func_working_mktime=yes jm_cv_func_working_re_compile_pattern=yes \
 		ac_use_included_regex=no gl_cv_c_restrict=no
 
+ifeq ($(BR2_TOOLCHAIN_HAS_THREADS),)
+	CAIRO_CONF_ENV += CPPFLAGS="$(TARGET_CPPFLAGS) -DCAIRO_NO_MUTEX=1"
+endif
+
 CAIRO_CONF_OPT = \
 	--enable-trace=no \
 	--enable-interpreter=no