scripts: support extra host compiler flags

Allow the user to configure extra flags to pass to the host compiler
at build time. Applies to both C and C++.

Useful on Ubuntu to turn off the stack protector and fortify defaults
so the program stands a better chance of running on other distros.

Signed-off-by: Michael Hope <michael.hope@linaro.org>
[yann.morin.1998@anciens.enib.fr: put the custom flags at the end]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

# Default test suite configuration

# Default DejaGnu configuration

DG_TOOLNAME = gcc

DG_TARGET_HOSTNAME = 127.0.0.1

DG_TARGET_USERNAME = root

# Default tests

DG_C_TESTS = 

DG_CPP_TESTS =