WIP: Add support for building subprojects with other build systems

[xclaesse]

This adds an experimental meson module to build projects with other
build systems (atm only configure/make). The project will be configured,
compiled and installed inside meson's build directory and during meson's
configuration phase.

[jpakkane]

The project will be configured, compiled and installed inside meson's build directory

Almost no project is relocatable. That is, they are only guaranteed to work if installed to their final resting place such as under /usr/local. If the files are anywhere else they are not guaranteed to work and can break in arbitrary ways.

Further this is an "environment setup" kind of action, not a build action. meson.build should not configure external build systems and, in fact, should not even know that such things exist.

[xclaesse]

@jpakkane When doing make DESTDIR=/path/to/meson/builddir install all files are in their final state, just not at their final location. So they are not in any "magical" state, we can totally use them. As far as I know the only limitation is we cannot run executables from there, but that's not a use-case I care about.

Again, here we don't have to provide any guarantee, we can just provide a tool that works in some limited scope. Like, for now, we can already use pkgconfig files from there. I don't think we should reject a feature that makes 90% of the use-cases easier, just because we could theoretically think about cases where it won't work.

Further this is an "environment setup" kind of action, not a build action. meson.build should not configure external build systems and, in fact, should not even know that such things exist.

Meson is about making life of developers easier. Ignoring the reality that not all project of the world use meson is not going to help anyone.

[xclaesse]

btw, if this is an "environment setup" kind of action, then we can just drop completely the whole concept of subprojects. That's a ton of code in meson that only setup your environment. You can just install each of them manually in the prefix.

[jpakkane]

As far as I know the only limitation is we cannot run executables from there, but that's not a use-case I care about.

Wouldn't the installed projects' pkg-config files point to the wrong place (i.e. the DESTDIRless install dir) and thus the output would not be usable because the libraries are not there?

[xclaesse]

That can be fixed by setting PKG_CONFIG_SYSROOT_DIR, my PR does that.

[xclaesse]

Ok, changed my mind about generating a meson.build from wrap file, it's too complicated. The wrap file could just have the path to a meson.build file to copy inside the subproject. I did that in a separate PR #4327.

[xclaesse]

Here is an example of meson.build file to build ffmpeg:

project('FFmpeg', [],
  meson_version : '>=0.48.9',
)

mod = import('unstable_external_project')

p = mod.add_project('configure',
  options : ['--prefix={prefix}',
             '--libdir={libdir}',
             '--incdir={includedir}'
            ],
)

libavfilter_dep = p.dependency('libavfilter')
ibavformat_dep = p.dependency('libavformat')
libavcodec_dep = p.dependency('libavcodec')
libavutil_dep = p.dependency('libavutil')

And this is to build AOM, a cmake project:

project('aom', [],
  meson_version : '>=0.48.9',
)

mod = import('unstable_external_project')

p = mod.add_project('cmake',
  options : ['-DCMAKE_INSTALL_PREFIX={prefix}',
             '-DCMAKE_INSTALL_LIBDIR={libdir}',
             '-DCMAKE_INSTALL_INCLUDEDIR={includedir}',
             '-DBUILD_SHARED_LIBS=1',
            ],
)

aom_dep = p.dependency('aom')

Both are working pretty well in gst-build.

[xclaesse]

With this extra patch, I successfully cross compiled ffmpeg for Android with gst-build.

[xclaesse]

Let's close this PR, I'll open a new one with revisited code.

[xclaesse]

For those interested, let's continue there: #6920.