Fix build with mingw

- include/libxml/xmlexports.h: restore export decoration otherwise
  xsltproc and xmlsec crash
- libxml.h: define LIBXML_STATIC for static build
- configure.in: enable modules support for mingw* builds
- Makefile.am: flags for testdso if modules support enabled

Makefile.am

@@ -159,7 +159,7 @@ testModule_LDADD= $(LDADDS)
 
 noinst_LTLIBRARIES = testdso.la
 testdso_la_SOURCES = testdso.c
-testdso_la_LDFLAGS = -module -rpath $(libdir)
+testdso_la_LDFLAGS = -module -no-undefined -avoid-version -rpath $(libdir)
 
 # that one forces the rebuild when "make rebuild" is run on doc/
 rebuild_testapi:

configure.in

@@ -779,6 +779,10 @@ if test "$with_modules" != "no" ; then
     AC_DEFINE([HAVE_DLOPEN], [], [Have dlopen based dso])
   ])
   ;;
+  *-*-mingw*)
+  MODULE_EXTENSION=".dll"
+  WITH_MODULES=1
+  ;;
   *)
   AC_CHECK_FUNC(shl_load, libxml_have_shl_load=yes, [
     AC_CHECK_LIB(dld, shl_load, [

include/libxml/xmlexports.h

@@ -113,7 +113,7 @@
    * _imp__xmlFree listed as missing. Try to workaround the problem
    * by also making that declaration when compiling client code.
    */
-  #if !defined(LIBXML_STATIC)
+  #if defined(IN_LIBXML) && !defined(LIBXML_STATIC)
     #define XMLPUBFUN __declspec(dllexport)
     #define XMLPUBVAR __declspec(dllexport)
   #else

libxml.h

@@ -90,4 +90,7 @@ void __xmlGlobalInitMutexDestroy(void);
 #endif
 #endif
 #endif
+#ifndef PIC
+#  define LIBXML_STATIC
+#endif
 #endif /* ! __XML_LIBXML_H__ */