testsuite: SKIP compile tests if configure found no compiler. * tests/depend6.test: configure will exit 77 if AC_PROG_CC found no working compiler. Allow the test to be SKIPped in that case. * tests/postproc.test: Likewise. * tests/pr243.test: Likewise. * tests/pr266.test: Likewise. Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>