libxml2: resolve missing dependencies when external libraries are needed for optional features

[Doekin]

This change enables iconv support by default for libxml2, aligning with its default configuration options.

This PR addresses an issue where enabling iconv can cause build failures for Android due to missing dependencies. The error typically appears as:

ld: error: undefined symbol: iconv
>>> referenced by encoding.c:1122 (/home/runner/.xmake/cache/packages/2504/l/libxml2/v2.14.0/source/encoding.c:1122)
>>>               encoding.c.o:(xmlIconvConvert) in archive /home/runner/.xmake/packages/l/libxml2/v2.14.0/cd35469623984bc18d849f390d29bfb2/lib/libxml2.a

[star-hengxing]

xrepo generally adheres to the principle of minimal dependencies and does not necessarily need to synchronize with upstream options.

[Doekin]

OK, I'll revert the last commit.

[luadebug]

Would it be convenient to define local packagedeps = {} and fill it accordingly to used options table.insert and pass it into (package, configs, {packagedeps = packagedeps})?
Reference https://github.com/xmake-io/xmake-repo/blob/dev/packages%2Fg%2Fgdal%2Fxmake.lua#L118

[Doekin]

Thank you for the suggestion. It should generally be fine to reference non-existing dependencies. If a library is not installed as a dependency, it won't take effect or cause errors during the build process. Therefore, passing all possible dependencies might be a simpler and more concise approach. However, I'm open to further discussion if you think a more selective approach would be better.

[waruqi]

We should pass dependencies as needed.

local packagedeps = {}
if package:config("zlib") then
    table.insert(packagedeps, "zlib")
end

In addition, packagedeps are usually not needed, because cmake will automatically find them through add_deps, and xmake will automatically set CMAKE_PREFIX_PATH and PKG_CONFIG_PATH to let cmake/pkg-config find these dependent libraries.

https://github.com/xmake-io/xmake/blob/1bc471e62072071c8cff6b9378d7460a1ce7aa85/xmake/modules/package/tools/cmake.lua#L1017-L1043

Only when cmake really cannot find a library, packagedeps will be used as a hack to support it.
So don't pass all packagedeps at once without any test verification.

[Doekin]

Understood. I'll investigate the root cause of the build failures for Android when enabling iconv, rather than using packagedeps without proper testing. Thank you for the guidance.