Add disable-static libtool flag

No one who builds harfbuzz static uses the autotools build system to do it.

configure.ac

diff --git a/configure.ac b/configure.ac
index 31b03d5..761a213 100644
--- a/configure.ac
+++ b/configure.ac
@@ -21,7 +21,7 @@
 
 # Initialize libtool
 LT_PREREQ([2.2])
-LT_INIT([])
+LT_INIT([disable-static])
 
 # Functions and headers
 AC_CHECK_FUNCS(mprotect sysconf getpagesize)