receiver: Makefile: Update compiler and linker flags
receiver: Makefile: Update compiler and linker flags

Compile with -fPIC rather than -fPIE, this seems right according to
https://fedoraproject.org/wiki/Changes/Harden_All_Packages and add
-Wl,-z,relro to LDFLAGS to ensure we get full RELRO support, this seems
to be needed with older compiler, e.g GCC 4.4.7 on RHEL 6.

This matches how we build receiptomatic-www

Signed-off-by: Andrew Clayton <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@digital-domain.net>

ac000/receiptomatic