configure: Show exhaustive tests in summary

Fixes bitcoin-core#1658.

Author: real-or-random

configure.ac

@@ -476,6 +476,7 @@ echo "Build Options:"
 echo "  with external callbacks = $enable_external_default_callbacks"
 echo "  with benchmarks         = $enable_benchmark"
 echo "  with tests              = $enable_tests"
+echo "  with exhaustive tests   = $enable_exhaustive_tests"
 echo "  with ctime tests        = $enable_ctime_tests"
 echo "  with coverage           = $enable_coverage"
 echo "  with examples           = $enable_examples"