tests: define only once the compiler and flag

[isidroas]

• tests: define only once the compiler and flag
  This way you could play with them easily. For example:

  • use clang instead gcc
  • -Wextra
  • -fsanitize=address

• check CC environ and fallback to gcc

[benhoyt]

Works for me, thanks!

$ time CC=gcc ./unittest.sh

real	0m0.841s
user	0m0.574s
sys	0m0.257s
$ time CC=tcc ./unittest.sh

real	0m0.141s
user	0m0.070s
sys	0m0.069s