configure.ac: update ldflags args

configure.ac: update ldflags args

Attach -Wl,-z,relro,-z,now,-z,pack-relative-relocs,--as-needed
to LDFLAGS, but only if LDFLAGS already doesn't contain 'relro'
(so it's not given repeatedly).

Also start to use -z,now linkage also when building libraries
with default compilation - this avoid calling symbol resolver
while library function are using function needing resolving.

Note: Fedora or RHEL rpm building is using:
CFLAGS=$(rpm --eval %{build_cflags})
LDFLAGS=$(rpm --eval %{build_ldflags})

Also split -DUSE_SD_NOTIFY into DEFS from CFLAGS.