package/cairo: mutex support depends on NPTL

Fixes:
http://autobuild.buildroot.net/results/041/041ac8f0809a0f9415c545b7585cd197db08b55f/

The last bump of cairo took place in 2019 with commit
8d2a9d089a969909ad2de8acb67ee14ced03de28, the first build error of this
kind I could find dates back to Sep 2022:
http://autobuild.buildroot.net/results/8c5/8c547ec2e148ce3c4f230bb4b0d5c5b360d74dd5/

Signed-off-by: Bernd Kuhls <bernd@kuhls.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 92f14307ba670058d0b3692fb8e82753d6ad6868)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/cairo/cairo.mk

@@ -26,7 +26,7 @@ ifeq ($(BR2_m68k_cf),y)
 CAIRO_CONF_ENV += CFLAGS="$(TARGET_CFLAGS) -mxgot"
 endif
 
-ifeq ($(BR2_TOOLCHAIN_HAS_THREADS),)
+ifeq ($(BR2_TOOLCHAIN_HAS_THREADS_NPTL),)
 CAIRO_CONF_ENV += CPPFLAGS="$(TARGET_CPPFLAGS) -DCAIRO_NO_MUTEX=1"
 endif