package/cairo: fix build with libexecinfo

Fix the following build failure raised on uclibc and musl since the
addition of libexecinfo package in commit
eea8ba446c10701a273432552108d80fb2224ef4:

/home/giuliobenetti/autobuild/run/instance-2/output-1/host/lib/gcc/arm-buildroot-linux-uclibcgnueabi/10.3.0/../../../../arm-buildroot-linux-uclibcgnueabi/bin/ld: /home/giuliobenetti/autobuild/run/instance-2/output-1/host/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libvcos.so: undefined reference to `backtrace_symbols'
/home/giuliobenetti/autobuild/run/instance-2/output-1/host/lib/gcc/arm-buildroot-linux-uclibcgnueabi/10.3.0/../../../../arm-buildroot-linux-uclibcgnueabi/bin/ld: /home/giuliobenetti/autobuild/run/instance-2/output-1/host/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libvcos.so: undefined reference to `backtrace'

Fixes:
 - http://autobuild.buildroot.org/results/836348270d564a46cc9ee840cf87b2494cc82cec

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/cairo/cairo.mk

@@ -15,6 +15,8 @@ CAIRO_INSTALL_STAGING = YES
 # 0002-ft-Use-FT_Done_MM_Var-instead-of-free-when-available-in-cairo_ft_apply_variation.patch
 CAIRO_IGNORE_CVES += CVE-2018-19876
 
+CAIRO_CONF_ENV = LIBS="$(CAIRO_LIBS)"
+
 # relocation truncated to fit: R_68K_GOT16O
 ifeq ($(BR2_m68k_cf),y)
 CAIRO_CONF_ENV += CFLAGS="$(TARGET_CFLAGS) -mxgot"
@@ -27,7 +29,7 @@ endif
 # cairo can use C++11 atomics when available, so we need to link with
 # libatomic for the architectures who need libatomic.
 ifeq ($(BR2_TOOLCHAIN_HAS_LIBATOMIC),y)
-CAIRO_CONF_ENV += LIBS="-latomic"
+CAIRO_LIBS += -latomic
 endif
 
 CAIRO_CONF_OPTS = \
@@ -81,6 +83,11 @@ else
 CAIRO_CONF_OPTS += --disable-ft
 endif
 
+ifeq ($(BR2_PACKAGE_LIBEXECINFO),y)
+CAIRO_DEPENDENCIES += libexecinfo
+CAIRO_LIBS += -lexecinfo
+endif
+
 ifeq ($(BR2_PACKAGE_LIBGLIB2),y)
 CAIRO_CONF_OPTS += --enable-gobject
 CAIRO_DEPENDENCIES += libglib2