Meson.build cleanup.

Meson.build cleanup.

Summary:
This is a 8 commit patch, but only for a while - after agreeing with
each of the changes, it shall be squashed into a single commit.

I want to make further changes on meson.build (maybe I can end up simplifying
the build system, or just let things more organized in the end) and thought
that starting with a cleanup would be a good first step.

The changes are:
1. build: set arguments scope to project instead of globally

   If we set arguments globally, it may conflict with other builds - specially
   considering [meson's subprojects
   feature](https://mesonbuild.com/Subprojects.html). Setting to project
   arguments ensures we are only considering EFL and not leaking unwanted flags.

2. build: Fix spacing and indent

   Mostly because it is not well standardized during the file - sometimes
   there's spaces between tokens, sometimes there is not, etc. The same applies
   to indent.

3. build: move test environment closer to test commands

   Just as a matter of organization. If we're doing things for tests that don't
   impact other stuff, then leave it when tests are handled.

4. build: Remove unnecessary parenthesis and == true comparisons

   Less noisy redundancy: `true` is already true, and `false` is already false,
   no need to re-check. Besides, reading `if sys_windows` and `if sys_windows
   == true` shouldn't have different effects, as the first you can read as "if
   the system is windows". It gets better when you have `not` instead of `==
   false`, so for an example you could read `if not sys_windows` as "if it is
   not a windows system" more naturally.

5. build: Switch pc_files to dict

   Just thought it could stay a little better (since it works as a dict),
   specially in the `foreach` right after.

6. **[removed to a future patch]** build: Use meson's warning_level instead of hardcoded -Wall

   This way we ensure this is compiler-independant (and use the correct feature
   for that, since meson even warns when configuring the build dir).

7. build: Use language args from add_project_arguments properly instead of a loop

   The `language:` kwarg from `add_{project,global}_arguments` receives a list
   of languages, so no need for that loop.

8. **[removed to a future patch]** build: Use '/' instead of join_paths

   As it [is recommended by meson since
   v0.49](https://mesonbuild.com/Release-notes-for-0-49-0.html#joining-paths-with-)
   (and stays clearer IMO, specially since that's how some languages are
   adopting path separation, e.g. C++'s filesystem stdlib).

Reviewers: bu5hm4n

Subscribers: vtorri, cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D11994