Support BSD in util Bash scripts

[lcheylus]

For Bash scripts in util/ (build-gnu.sh, run-gnu-test.sh, build-code_coverage.sh...), commands for make and readlink are not compatible with BSD OS (FreeBSD, NetBSD and OpenBSD).

• GNU make, not compatible with make on BSD
• readlink uses -m flag : not present for readlink on BSD, only in GNU coreutils readlink

On BSD:

• GNU make could be installed via package (gmake on OpenBSD)
• GNU coreutils (readlink) is available in coreutils package on OpenBSD

Proposal to modify Bash scripts to be compatible with BSD OS and Linux (with GNU coreutils installed by default)

• checks OS and defines MAKE/READLINK variables

# Check OS to use GNU coreutils and make: specific version on *BSD
case "$OSTYPE" in
    *bsd*)
        MAKE="gmake"
        READLINK="greadlink"
        ;;
    *)
        MAKE="make"
        READLINK="readlink"
        ;;
esac

• replace make by ${MAKE}
• replace readlink by ${READLINK}

[sylvestre]

Sure, don't hesitate to submit a PR