libxml configure bugs

[Drazen Kacar <dave srce hr>]

There are several bugs in libxml 1.0 configure script and generated Makefiles.
It doesn't check CPPFLAGS environment variable, so it fails to find zlib
when it's in non-standard place, because configure test is run with
cpp without any flags specified by the user.

Makefile generated in doc subdirectory doesn't always use $(INSTALL) as
an instalation command. When I changed that, install-sh was called with
several files to install in one invocation, but the script can't handle
that.

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