Problem: test_fork causes valgrind false positive

Solution: do not run test_fork if --enable-valgrind is set. Note that
later versions of Valgrind (3.11) not yet available in all
distributions fix this problem, so we might revert in the future.

Makefile.am

@@ -657,11 +657,13 @@ tests_test_use_fd_tcp_SOURCES = \
 tests_test_use_fd_tcp_LDADD = src/libzmq.la
 
 if HAVE_FORK
+if !VALGRIND_ENABLED
 test_apps += tests/test_fork
 
 tests_test_fork_SOURCES = tests/test_fork.cpp
 tests_test_fork_LDADD = src/libzmq.la
 
+endif
 endif
 endif
 endif