Added support for marking heap blocks as inaccessible/uninitialized if
valgrind is installed, based on a patch by Adam Gundy.

diff --git a/configure.ac b/configure.ac
index 649a9da..f6c2ab8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1072,6 +1072,7 @@
 	termios.h \
 	unistd.h \
 	utime.h \
+	valgrind/memcheck.h
 )
 AC_HEADER_STAT()