Misc fixes: build issues on windows, improve bootstrap parallelization, python3.5 compat, etc

commit 25ef134e:

cerbero: Remove some extremely outdated distro support

commit e3c4b0d1:

libiconv.recipe: Remove libtool dependency
When building as a build-tool, don't override ltmain.sh since that
forces a libtool dependency.

commit f857d54e:

libiconv.recipe: pls no commenting things out

commit 184ebc7a:

cerbero: Simplify some job allocation code

commit ad64728e:

gettext-tools.recipe: Remove libtool dependency
Don't override ltmain.sh since that forces a libtool dependency, which
we do not need for build-tools.

commit 73b0c8c8:

cerbero: Increase parallel builds on Windows
We run make with -j1 on Windows due to make bugs, so we can run more
build steps for recipes in parallel without contention.

However, we cannot use `self.jobs` as the build job limit because a lot
of CPU is used by the Windows Defender anti-virus which tracks the
work done by build system processes even if you add the cerbero build
directory to the exclusion list.

commit 2db94179:

pkg-config.recipe: Remove libtool dependency
Don't override ltmain.sh since that forces a libtool dependency, which
we do not need for build-tools.

commit 42db9ec6:

cerbero: Fix setting of git credentials
The git credentials need to be set in the repo we're cloning into, not
the repo we're cloning from.

commit f3bb6cbb:

cerbero: ModuleNotFoundError was added in Python 3.6
And we require Python 3.5

commit 8f615a52:

cerbero: await in list comprehensions is not allowed in Python 3.5
`SyntaxError: 'await' expressions in comprehensions are not supported`

commit e14af38f:

Replace 3.7 create_task with 3.5.1 ensure_future
create_task is only available in 3.7, and in 3.8 the API
changes. So, we'd rather use the ensure_future since it
does exactly the same even though the docs say it may be
less readable.

commit 06ca5bd9:

recipe/post_install: Convert check_call to check_output
This is nicer, and less likely to have shell quoting issues.

commit c962311a:

cerbero: Fix builds with CMake recipes on Windows
If there's a leading space in `LDFLAGS`, that somehow causes CMake to
convert `-Lc:/path/to/cerbero/build/dist/windows_x86_64/lib`
to `-Lc;C:\MinGW\msys\1.0\path\to\cerbero\build\dist\windows_x86_64\lib`.

`;` is the end of that command and then the rest gets interpreted as a
new command to run: `C:MinGWmsys1.0pathtocerberobuilddistwindows_x86_64lib`

Not sure how this all happens; it's happening somewhere inside the
CMake source code, and I don't want to delve into that.

Fixes https://gitlab.freedesktop.org/gstreamer/cerbero/issues/199

commit 791ffcc1:

srt.recipe: Restore usage of public API from MSVC
Not sure why upstream is still putting this in here. Maybe because
they did it in the past and apps will break or something?

Fixes https://gitlab.freedesktop.org/gstreamer/cerbero/issues/201