gdb: linking workaround for multiple definitions

Multiple defintiions for some symbols such as PC are introduced
by readline and termcap. This causes GCC 10 linking error.

Related to:
    Default to -fno-common
    https://gcc.gnu.org/gcc-10/porting_to.html

Credit:
    https://github.com/Ettercap/ettercap/issues/795

src/gdb.mk

@@ -24,7 +24,8 @@ define $(PKG)_BUILD
         --disable-gdbtk \
         --disable-tui \
         host_configargs="LIBS=\"`$(TARGET)-pkg-config --libs dlfcn` -lmman\"" \
-        CONFIG_SHELL=$(SHELL)
+        CONFIG_SHELL=$(SHELL) \
+        LDFLAGS='-Wl,--allow-multiple-definition'
     $(MAKE) -C '$(BUILD_DIR)' -j '$(JOBS)'
 
     # executables are always static and we don't want the rest