Merge branch 'compilers-for-testsuite'

* compilers-for-testsuite:
  readme: how to run the testsuite with cross-compilers
  tests: no need to unset CFLAGS in tests requiring 'gcc' anymore
  test defs: allow compilers to be auto-selected on user's request
  test defs: substitute compilers and flags found at configure time
  test defs: setup `*FLAGS' variables for GNU compilers
  configure: search generic compilers for use in the tests

diff --cc tests/Makefile.am
Simple merge

diff --cc tests/defs
Simple merge