build: Allow augmenting TESTS_ENVIRONMENT
Add substitution of a special variable "AUX_TESTS_ENVIRONMENT" to the
"TESTS_ENVIRONMENT" value, allowing its augmentation from the make
command line. This enables wrapping test commands with older versions of
Automake, where LOG_COMPILER support is missing.
This enables executing "make check" target with Valgrind on RHEL6, like
this:
make check AUX_TESTS_ENVIRONMENT="libtool --mode=execute valgrind"
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>