xbps-src: set default compiler/preprocessor/linker flags via etc/conf.

Rather than setting them in the build profile, just set them in
etc/defaults.conf (overridable via etc/conf) to simplify the code.

- Also enable _FORTIFY_SOURCE=2 in CPPFLAGS.
- Also enable -z,relro in LDFLAGS.

As suggested by @chneukirchen.

etc/defaults.conf

@@ -24,15 +24,16 @@ XBPS_INSTALL_ARGS="--repository=http://repo.voidlinux.eu/current"
 # Native Compilation flags for C and C++. Note that these settings override
 # the pre-defined for the target architecture in common/build-profiles/arch.sh.
 #
-#XBPS_CFLAGS="-O2 -pipe -fstack-protector-strong"
-#XBPS_CXXFLAGS="${XBPS_CFLAGS}"
+XBPS_CFLAGS="-O2 -pipe -fstack-protector-strong"
+XBPS_CXXFLAGS="${XBPS_CFLAGS}"
+XBPS_CPPFLAGS="-D_FORTIFY_SOURCE=2"
 
 # [OPTIONAL]
 # Linker flags passed to the compiler. By default we use --as-needed to
 # avoid linking extra libraries into binaries. See the following link
 # for info: http://www.gentoo.org/proj/en/qa/asneeded.xml
 #
-XBPS_LDFLAGS="-Wl,--as-needed"
+XBPS_LDFLAGS="-Wl,--as-needed -Wl,-z,relro"
 
 # [REQUIRED]
 # Command to execute to gain root privileges when using the `update-sys`