package/grpc: fix build with libexecinfo

Fix the following build failure raised on uclibc and musl since the
reintroduction of the package in commit
16ff948444:

/home/buildroot/autobuild/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/arm-buildroot-linux-uclibcgnueabi/10.3.0/../../../../arm-buildroot-linux-uclibcgnueabi/bin/ld: /home/buildroot/autobuild/instance-1/output-1/host/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libabsl_stacktrace.so.2111.0.0: undefined reference to `backtrace'

Fixes:
 - http://autobuild.buildroot.org/results/63ab2bc86cad03d5258492b17d1707078761d9b3

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

package/grpc/grpc.mk

@@ -21,6 +21,7 @@ HOST_GRPC_DEPENDENCIES = host-protobuf
 # which doesn't do this.  These CARES settings trick the gRPC cmake code into
 # not looking for c-ares at all and yet still linking with the library.
 GRPC_CONF_OPTS = \
+	-DCMAKE_EXE_LINKER_FLAGS="$(GRPC_EXE_LINKER_FLAGS)" \
 	-DgRPC_ABSL_PROVIDER=package \
 	-D_gRPC_CARES_LIBRARIES=cares \
 	-DgRPC_CARES_PROVIDER=none \
@@ -36,10 +37,15 @@ GRPC_CONF_OPTS = \
 	-DgRPC_BUILD_GRPC_PYTHON_PLUGIN=OFF \
 	-DgRPC_BUILD_GRPC_RUBY_PLUGIN=OFF
 
+ifeq ($(BR2_PACKAGE_LIBEXECINFO),y)
+GRPC_DEPENDENCIES += libexecinfo
+GRPC_EXE_LINKER_FLAGS += -lexecinfo
+endif
+
 # grpc can use __atomic builtins, so we need to link with
 # libatomic when available
 ifeq ($(BR2_TOOLCHAIN_HAS_LIBATOMIC),y)
-GRPC_CONF_OPTS += -DCMAKE_EXE_LINKER_FLAGS=-latomic
+GRPC_EXE_LINKER_FLAGS += -latomic
 endif
 
 GRPC_CFLAGS = $(TARGET_CFLAGS)