The libxml extension is enabled by default, although it may be disabled with --without-libxml.
PHP uses pkg-config to select the right library file, header files, and compile flags to use for libxml2. To ensure that the desired version of libxml2 is selected, the PKG_CONFIG_PATH environment variable may be used to control the search path for pkg-config prior to running the configure script:
PKG_CONFIG_PATH="/path/to/libxml2/prefix/lib/pkgconfig:/lib/pkgconfig"