package/i2pd: link with libatomic when needed

On some architectures, atomic binutils are provided by the libatomic
library from gcc. Linking with libatomic is therefore necessary,
otherwise the build fails with:

sparc-buildroot-linux-uclibc/sysroot/lib/libatomic.so.1: error adding symbols: DSO missing from command line

This is often for example the case on sparcv8 32 bit.

Fixes:
 - http://autobuild.buildroot.org/results/515160349b11f06a090f3e13992e30da9a402c17

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

package/i2pd/i2pd.mk

@@ -21,6 +21,10 @@ I2PD_CONF_OPTS += -DWITH_GUI=OFF
 # build failure when cross-compiling.
 I2PD_CONF_OPTS += -DTHREADS_PTHREAD_ARG=OFF
 
+ifeq ($(BR2_TOOLCHAIN_HAS_LIBATOMIC),y)
+I2PD_CONF_OPTS += -DCMAKE_CXX_FLAGS="$(TARGET_CXXFLAGS) -latomic"
+endif
+
 ifeq ($(BR2_STATIC_LIBS),y)
 I2PD_CONF_OPTS += -DWITH_STATIC=ON
 else