Improve Vala support

[al1-ce]

When compiling multiple files valac fails to find symbols (i.e SomeClass from a.vala using in b.vala), changed build to process all source files at once

[waruqi]

Did you test other examples.

e.g. https://github.com/xmake-io/xmake/tree/dev/tests/projects/vala/staticlib

it does not work.

and others. https://github.com/xmake-io/xmake/tree/dev/tests/projects/vala

[al1-ce]

Checking rn

[al1-ce]

Strange, vala/lua test doesn't compile even with upstream rule file...

[al1-ce]

Implemented requested changes, all tests passing except for lua, but lua fails because of xmake (and maybe arch linux, it has headers who knows where) coz gcc fails to find lua includes from -Llua flag

Also fixed vala.vapidir and vala.flags not being applied and fixed vala.vapifile and vala.header locations being incorrect

[al1-ce]

Couldn't get valac build to run only when needed, but everything else should be correct now

[waruqi]

But incremental compilation still doesn't work.

bash-5.2$ xmake
[ 20%]: compiling.vala src/mymath.vala
[ 70%]: compiling.vala src/main.vala
[100%]: build ok, spent 0.588s

bash-5.2$ xmake
[ 20%]: compiling.vala src/mymath.vala
[ 70%]: compiling.vala src/main.vala
[100%]: build ok, spent 0.571s

[al1-ce]

Oh, I think I'm starting to understand how batchcmds work, not completely though...
Hard to figure out, not that much documentation about it, except for examples which do not explain anything

[waruqi]

Oh, I think I'm starting to understand how batchcmds work, not completely though... Hard to figure out, not that much documentation about it, except for examples which do not explain anything

https://xmake.io/#/manual/custom_rule?id=custom-batch-compile-script-process-one-source-file-at-a-time

[al1-ce]

https://xmake.io/#/manual/custom_rule?id=custom-batch-compile-script-process-one-source-file-at-a-time

Again, shows how, but now why. Maybe worth adding some explaining comments to it?

Anyway, thanks for taking time to review and merge my PR