ENH: support shared libraries inside packages

This drops the use of staged installs with `--destdir` in favor
of using `--prefix` for the install location. The problem is that
RPATHs in extension modules end up pointing to what Meson thinks is
the final install location (i.e., inside the prefix) rather than
to the staging directory.

`--destdir` seems meant for packaging, as an actual staging area,
while for `spin` the final install directory is `build-install`
(by default), there is no intent to later put this package into
`/usr` or `C:\`. Hence using `--prefix` seems like the correct
thing to do.

The one test change here is to a test that was incorrect.
`meson setup --prefix` expects an absolute path, and `/foobar`
isn't a path that exists or can be created.

Addresses the issue discussed in PR 238 - the `spin build` behavior
before this change cannot be made to work for SciPy, because the
internal shared library in `scipy.special` keeps breaking.

It should also address the problem discussed in issue spin#176.

spin/cmds/meson.py

@@ -231,11 +231,6 @@ def _check_coverage_tool_installation(coverage_type: GcovReportFormat, build_dir
         )
 
 
-if sys.platform.startswith("win"):
-    DEFAULT_PREFIX = "C:/"
-else:
-    DEFAULT_PREFIX = "/usr"
-
 build_dir_option = click.option(
     "-C",
     "--build-dir",
@@ -266,10 +261,10 @@ def _check_coverage_tool_installation(coverage_type: GcovReportFormat, build_dir
 )
 @click.option(
     "--prefix",
-    help="The build prefix, passed directly to meson.",
+    help="The build prefix as an absolute path, passed directly to meson.",
     type=str,
     metavar="PREFIX",
-    default=DEFAULT_PREFIX,
+    default='',
 )
 @click.argument("meson_args", nargs=-1)
 @build_dir_option
@@ -320,6 +315,9 @@ def build(
     install_dir = _get_install_dir(build_dir)
     abs_install_dir = os.path.abspath(install_dir)
 
+    if not prefix:
+        prefix = abs_install_dir
+
     cfg = get_config()
     distname = cfg.get("project.name", None)
     if distname and _is_editable_install_of_same_source(distname):
@@ -385,10 +383,6 @@ def build(
             "--only-changed",
             "-C",
             build_dir,
-            "--destdir",
-            install_dir
-            if os.path.isabs(install_dir)
-            else os.path.relpath(abs_install_dir, abs_build_dir),
         ]
         + list(meson_install_args),
         output=(not quiet) and verbose,

spin/tests/test_build_cmds.py

@@ -38,7 +38,8 @@ def test_debug_builds(example_pkg):
 
 def test_prefix_builds(example_pkg):
     """does spin build --prefix create a build-install directory with the correct structure?"""
-    spin("build", "--prefix=/foobar/")
+    prefix = Path('.').resolve() / 'build-install' / 'foobar'
+    spin("build", f"--prefix={prefix}")
     assert (Path("build-install") / Path("foobar")).exists()