Various "make distcheck" and portability fixups

Makefile.am:

* Don't use @VAR@, use $(VAR). Autoconf's AC_SUBST provides us the Make
   variable, it allows overriding the value at the command line, and
   (notably) it avoids a Make parse error in the libxml2_la_LDFLAGS
   assignment when @MODULE_PLATFORM_LIBS@ is empty

* Changed how the THREADS_W32 mechanism switches the build between
   testThreads.c and testThreadsWin32.c as appropriate; using AM_CONDITIONAL
   allows this to work cleanly and plays well with dependencies

* testapi.c should be specified as BUILT_SOURCES

* Create symlinks to the test/ and result/ subdirs so that the runtests
   target is usable in out-of-source-tree builds

* Don't do MAKEFLAGS+=--silent as this is not portable to non-GNU Makes

* Fixed incorrect find(1) syntax in the "cleanup" rule, and doing "rm -f"
   instead of just "rm" is good form

* (DIST)CLEANFILES needed a bit more coverage to allow "make distcheck" to
   pass

configure.in:

* Need AC_PROG_LN_S to create test/ and result/ symlinks in Makefile.am

* AC_LIBTOOL_WIN32_DLL and AM_PROG_LIBTOOL are obsolete; these have been
   superceded by LT_INIT

* Don't rebuild docs by default, as this requires GNU Make (as
   implemented)

* Check for uint32_t as some platforms don't provide it

* Check for some more functions, and undefine HAVE_MMAP if we don't also
   HAVE_MUNMAP (one system I tested on actually needed this)

* Changed THREADS_W32 from a filename insert into an Automake conditional

* The "Copyright" file will not be in the current directory if builddir !=
   srcdir

doc/Makefile.am:

* EXTRA_DIST cannot use wildcards when they refer to generated files; this
   breaks dependencies. What I did was define EXTRA_DIST_wc, which uses GNU
   Make $(wildcard) directives to build up a list of files, and EXTRA_DIST,
   as a literal expansion of EXTRA_DIST_wc. I also added a new rule,
   "check-extra-dist", to simplify checking that the two variables are
   equivalent. (Note that this works only when builddir == srcdir)

   (I can implement this differently if desired; this is just one way of
   doing it)

* Don't define an "all" target; this steps on Automake's toes

* Fixed up the "libxml2-api.xml ..." rule by using $(wildcard) for
   dependencies (as Make doesn't process the wildcards otherwise) and
   qualifying appropriate files with $(srcdir)

   (Note that $(srcdir) is not needed in the dependencies, thanks to VPATH,
   which we can count on as this is GNU-Make-only code anyway)

doc/devhelp/Makefile.am:

* Qualified appropriate files with $(srcdir)

* Added an "uninstall-local" rule so that "make distcheck" passes

doc/examples/Makefile.am:

* Rather than use a wildcard that doesn't work, use a substitution that
   most Make programs can handle

doc/examples/index.py:

* Do the same here

include/libxml/nanoftp.h:

* Some platforms (e.g. MSVC 6) already #define INVALID_SOCKET:

     user@host:/cygdrive/c/Program Files/Microsoft Visual Studio/VC98/\
     Include$ grep -R INVALID_SOCKET .
     ./WINSOCK.H:#define INVALID_SOCKET  (SOCKET)(~0)
     ./WINSOCK2.H:#define INVALID_SOCKET  (SOCKET)(~0)

include/libxml/xmlversion.h.in:

* Support ancient GCCs (I was actually able to build the library with 2.5
   but for this bit)

python/Makefile.am:

* Expanded CLEANFILES to allow "make distcheck" to pass

python/tests/Makefile.am:

* Define CLEANFILES instead of a "clean" rule, and added tmp.xml to allow
   "make distcheck" to pass

testRelax.c:

* Use HAVE_MMAP instead of the less explicit HAVE_SYS_MMAN_H (as some
   systems have the header but not the function)

testSchemas.c:

* Use HAVE_MMAP instead of the less explicit HAVE_SYS_MMAN_H

testapi.c:

* Don't use putenv() if it's not available

threads.c:

* This fixes the following build error on Solaris 8:

     libtool: compile:  cc -DHAVE_CONFIG_H -I. -I./include -I./include \
     -D_REENTRANT -D__EXTENSIONS__ -D_REENTRANT -Dsparc -Xa -mt -v \
     -xarch=v9 -xcrossfile -xO5 -c threads.c  -KPIC -DPIC -o threads.o
     "threads.c", line 442: controlling expressions must have scalar type
     "threads.c", line 512: controlling expressions must have scalar type
     cc: acomp failed for threads.c
     *** Error code 1

trio.c:

* Define isascii() if the system doesn't provide it

trio.h:

* The trio library's HAVE_CONFIG_H header is not the same as LibXML2's
   HAVE_CONFIG_H header; this change is needed to avoid a double-inclusion

win32/configure.js:

* Added support for the LZMA compression option

win32/Makefile.{bcb,mingw,msvc}:

* Added appropriate bits to support WITH_LZMA=1

* Install the header files under $(INCPREFIX)\libxml2\libxml instead of
   $(INCPREFIX)\libxml, to mirror the install location on Unix+Autotools

xml2-config.in:

* @MODULE_PLATFORM_LIBS@ (usually "-ldl") needs to be in there in order for
   `xml2-config --libs` to provide a complete set of dependencies

xmllint.c:

* Use HAVE_MMAP instead of the less-explicit HAVE_SYS_MMAN_H