external-project: Add devenv support

[xclaesse]

No description provided.

[MaxandreOgeret]

Solves the issue I had when dynamically linking against a library built in an "external-project":

subproject meson.build:

project('ctemplate', 'cpp',
        meson_version : '>=0.56.0',
)

mod = import('unstable-external_project')

run_command('sh', './autogen.sh', check : true)

p = mod.add_project('configure',
                    configure_options : ['--prefix=@PREFIX@',
                                         '--libdir=@PREFIX@/@LIBDIR@',
                    ],
)

lib_dir = join_paths(meson.current_build_dir(), 'lib')
ctemplate_dep = p.dependency('ctemplate')
meson.override_dependency('ctemplate', ctemplate_dep)

Main project meson.build:

project('CTemplateTest', 'cpp',
        version : '1.0.0',
        default_options : ['warning_level=3', 'cpp_std=c++20'])

ctemplate_proj = subproject('ctemplate')
ctemplate_dep = dependency('ctemplate')

CTemplateTest = executable('CTemplateTest', 'main.cpp',
                           dependencies : [ctemplate_dep],
                           install : true)

[dcbaker]

One teeny nit, otherwise this looks fine to me.