m4/valgrind-tests.m4: add valgrind testing support to automake
.travis.yml: add a travis build entry to run tests under valgrind
Makefile.am: add valgrind to the AM_TESTS_ENVIRONMENT
configure.ac: enable valgrind testing option for ./configure
m4/valgrind-tests.m4: enable tracing children for libtool wrapper
script compatibility, but ignore standard binaries in /usr or /bin
.travis.yml: install valgrind on the CI VM