This is patch that improve libxml mingw build that use GNU autotools
and make it more compatible to existing win32 builds.
Build is done with "gcc 3.4.5 (mingw special)".
The patch is for version 2.6.27 but can be applied to 2.6.29.

Changes:
- ./configure.in
  - Sset flags for mingw build to enable modules, modules extention and ld flag 
    "-avoid-version" is to create dll with name libxml2.dll instead of
    libxml2[versufix].dll where [versufix] is -2 for version info 8:27:6 (=8-6).
- ./Makefile.am
  - Add XXX_EXTRA_LDFLAGS to testdso otherwise shared
  library isn't not created. Flag -no-undefined should present for win32 builds and
  for -avoid-version in case of mingw see above.
  - Remove $(EXEEXT) from test scripts. Libtool create wrapper script without .exe suffix.
  This script set paths needed to run exutable in .libs directory. If test script
  run directly program$(EXEEXT), program fail since cannot load shared libraries.
- ./doc/devhelp/Makefile.am
  Changes are not related to mingw build. Modification fix problem when builddir !=
sourcedir.
- ./error.c
  After modification error message will contain "input file name", "line number" and
  other information in case of mingw build.
  Lets see test case .build/xmllint --noout --relaxng ./test/relaxng/tutorA.rng ./test/relaxng/tutor9_5_2.xml
  Expacted first line in error message is:
  "./test/relaxng/tutor9_5_2.xml:1: element addressBook: Relax-NG validity error : Did not expect element addressBook there"
  Without modification the line is:
  "Did not expect element addressBook there"
  ,i.e. without file, line, (?), type(?).

  The pointer to function fprintf in shared library(dll) differ from
  pointer to same function in executable. This is true for mingw.
  If I remember well this is true for borland compiler since I/O stream
  function are part for CRT. It is posible in recent compiler versions
  this to be solved. I test Igors build (msc 13) and its is ok.

  I'm not sure that my modification is correct. I dont understand why should call
  channel if it differ from fprintf or xmlGenericErrorDefaultFunc.

  I think that comparison chanel == fprintf is not correct for win32 platform.
  It's seems to me that return is true only if dll and program are build with
  msc compiler. If dll is build with one compiler but program with other or 
  both with gcc(mingw), borland and may other I expert result to be false.


With patch above I'm able to run all tests under wine simulation
and results are same as linux.

I don't resolve one problems. When I run mingw build on win32 platform following test: 
"xmllint.exe --noout --relaxng .\tutorA.rng .\tutor9_5_2.xml"
the error message is:
".%5Ctutor9_5_2.xml:1: element addressBook: Relax-NG validity error : Did not ex
pect element addressBook there
.\tutor9_5_2.xml fails to validate"

, i.e. %5C instead \. Under wine error message is OK.