Use fseeko and ftello on BSDs

Add defined(ANDROID) for fseeko defined and android-x86_64

meson.build: fix typo

Modern CMake overhaul and project fixes.

Fixes Query results in xmltest being assigned to int instead of XMLError

Fix type in meson.build

Fixed typo

Add meson build system

removed overhead of clear error

cmake: Use a namespace for the tinyxml2 target in local export

Fix for enum in StrPair and TIXMLASSERT define behaviour

Add missing `defined`, detected by -Werror=undef

skip comment node before get text

Fix errors/warnings with hexadecimal format string in sscanf

Meson is a build system somewhat like cmake, but without all of the
rough edges. It supports many OSes, including all of the major ones, and
a large number of C++ compilers.

My interest isn't really in convincing people to use meson as the
default here, but meson provides a subproject mechanism that can fetch
external projects and build them along with the main project in a single
configure/compile invocation. This is extremely useful for platforms
that lack a (competent) package manager.

As far as I know the meson build does everything the cmake build does,
with one exception: generate the cmake config/version files. meson can
generate these files, but only in simple cases, and not when using
export targets like tinyxml2 does.

Previously in the locally (cmake build dir) exported cmake script,
there was no namespace, so the targets would directly be available
in the global namespace. Users would do something like:

    include(cmake/tinyxml2Targets.cmake)
    target_link_libraries(main tinyxml2)

Now, a namespace is used (just like the cmake export that is exported
to the system at the `install(EXPORT...)` command at the bottom of this
file. Uses will now look like:

    include(cmake/tinyxml2Targets.cmake)
    target_link_libraries(main tinyxml2::tinyxml2)

This is technically a minor breaking change. It will only affect users
that actually use the *locally* export cmake targets script, which I
expect to be fairly few (note that this is different from the system
exported script). Also, it will only affect users that freshly
build at this commit of tinyxml2, and have the cmake for their
downstream project configured to load it. That cmake will need to be
changed from the first snippet to the second snippet above.

cmake: Fix debug postfix in pkgconfig file

cmake: Refactor targets export variable and improve comments

cmake: Don't export symbols if building static library

- Fix for enum bitmask usage