Fix libz and liblzma detection
If libz or liblzma are detected with pkg-config, AC_CHECK_HEADERS must not be run because the correct CPPFLAGS aren't set. It is actually not required have separate checks for LIBXML_ZLIB_ENABLED and HAVE_ZLIB_H. Only check for LIBXML_ZLIB_ENABLED and remove HAVE_ZLIB_H macro. Fixes bug 764657, bug 787041.
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