Feature: let the user add custom LDFLAGS to the main Makefile
Feature: let the user add custom LDFLAGS to the main Makefile

For instance, with archlinux makepkg, it is good practice to compile
binaries with full RELRO (-Wl,-z,relro,-z,now). Otherwise, namcap
complains about it. This addition enables the additional flags to be
added via the user environment or building tools.

seladb/PcapPlusPlus,seladb/PcapPlusPlus,seladb/PcapPlusPlus,seladb/PcapPlusPlus