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
.travis.yml: add VERBOSE=1 to make flags in order to log test failures

"As with the serial harness above, by default one status line is printed
per completed test, and a short summary after the suite has completed.
However, standard output and standard error of the test are redirected
to a per-test log file, so that parallel execution does not produce
intermingled output. The output from failed tests is collected in the
test-suite.log file. If the variable ‘VERBOSE’ is set, this file is
output after the summary."

(http://www.gnu.org/software/automake/manual/html_node/Parallel-Test-Harness.html)
Makefile.am: AM_TESTS_ENVIRONMENT -> LOG_COMPILER

    It’s important to note that, differently from what we’ve seen for
    the serial test harness (see Parallel Test Harness), the
    AM_TESTS_ENVIRONMENT and TESTS_ENVIRONMENT variables cannot be use
    to define a custom test runner; the LOG_COMPILER and LOG_FLAGS (or
    their extension-specific counterparts) should be used instead:

    ## This is WRONG!
    AM_TESTS_ENVIRONMENT = PERL5LIB='$(srcdir)/lib' $(PERL) -Mstrict -w

    ## Do this instead.
    AM_TESTS_ENVIRONMENT = PERL5LIB='$(srcdir)/lib'; export PERL5LIB;
    LOG_COMPILER = $(PERL)
    AM_LOG_FLAGS = -Mstrict -w

(http://www.gnu.org/software/automake/manual/html_node/Parallel-Test-Harness.html)
.travis.yml: remove "make clean" at end of build script
.travis.yml: add back "language: c"

of course, protobuf-c uses both C and C++.