package/boinc: fix build with libexecinfo

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

/home/buildroot/autobuild/instance-0/output-1/host/opt/ext-toolchain/bin/../lib/gcc/arc-buildroot-linux-uclibc/10.2.0/../../../../arc-buildroot-linux-uclibc/bin/ld: ../lib/.libs/libboinc.a(libboinc_la-diagnostics.o): in function `boinc_catch_signal':
diagnostics.cpp:(.text+0x8a): undefined reference to `backtrace'

Fixes:
 - http://autobuild.buildroot.org/results/4504379b464eb144a4c257001eb4d316bb1f5e44

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/boinc/boinc.mk

@@ -39,7 +39,12 @@ ifeq ($(BR2_PACKAGE_FREETYPE),y)
 BOINC_DEPENDENCIES += freetype
 endif
 
-BOINC_MAKE_OPTS = CXXFLAGS="$(TARGET_CXXFLAGS) -std=c++11"
+ifeq ($(BR2_PACKAGE_LIBEXECINFO),y)
+BOINC_DEPENDENCIES += libexecinfo
+BOINC_MAKE_OPTS += LIBS="-lexecinfo"
+endif
+
+BOINC_MAKE_OPTS += CXXFLAGS="$(TARGET_CXXFLAGS) -std=c++11"
 
 # Remove boinc-client because it is incompatible with buildroot
 define BOINC_REMOVE_UNNEEDED_FILE