Make dynamic libraries under MSYS

Libtool did not make DLL when building under MSYS.
Signed-off-by: Jean-Paul Saman <jean-paul.saman@m2x.nl>
(cherry picked from commit 3f8a2b7c)
(cherry picked from commit e3cbc9d3)

configure.ac

@@ -12,6 +12,7 @@ AC_PROG_CC
 AC_STDC_HEADERS
 AC_C_INLINE
 
+AC_LIBTOOL_WIN32_DLL
 AM_PROG_LIBTOOL
 
 dnl default CFLAGS