[DVDnav-discuss] libdvdread uses internal dlfcn on W32, unconditionally

[LRN]

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On W32 libdvdread unconditionally uses msvc/contrib/dlfcn.c
While this allows libdvdread to be compiled out of the box, it
prevents it from linking to any other dlfcn implementation. Namely -
to dlfcn-win32 [1] (which is somewhat more POSIX-compliant).

The attached patch is very simplistic (i.e. it unconditionally
requires a working libdl somewhere), but it works for me. For trunk i
would suggest re-writing it like this:
check for dlopen in -lc
  if that fails, check for dlopen in -ld
    if that fails AND host is mingw, modify CFLAGS to use internal dlfcn

Note that LDFLAGS modification should go AFTER (!) all AC_CHECK_LIB
calls, because -no-undefined is no longer valid as a compiler option.

[1] http://code.google.com/p/dlfcn-win32/
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--- libdvdread-4.2.0/configure.ac.orig	2012-04-13 19:55:31 +0400
+++ libdvdread-4.2.0/configure.ac	2012-04-13 19:55:45 +0400
@@ -139,8 +139,13 @@
 dnl ---------------------------------------------
 case $host in
   *mingw32*)
-    CFLAGS="-idirafter \$(top_srcdir)/msvc/include $CFLAGS"
+    AC_CHECK_LIB(c, dlopen,
+                 DYNAMIC_LD_LIBS="",
+                 AC_CHECK_LIB(dl, dlopen,
+                              DYNAMIC_LD_LIBS="-ldl",
+                              AC_MSG_ERROR(dynamic linker needed)))
+    AC_SUBST(DYNAMIC_LD_LIBS)
     LDFLAGS="-no-undefined $LDFLAGS"
     ;;
   *cygwin*)
     LDFLAGS="-no-undefined $LDFLAGS"