Support absolute bindir/libdir in regression tests with meson

Passing an absolute bindir/libdir will install the binaries and
libraries to <build>/tmp_install/<bindir> and
<build>/tmp_install/<libdir> respectively.

This path is correctly passed to the regression test suite via
configure/make, but not via meson, yet. This is because the "/"
operator in the following expression throws away the whole left side
when the right side is an absolute path:

    test_install_location / get_option('libdir')

This was already correctly handled for dir_prefix, which is likely
absolute as well. This patch handles both bindir and libdir in the
same way - prefixing absolute paths with the tmp_install path
correctly.

Author: Wolfgang Walther
Reviewed-by: Nazir Bilal Yavuz, Alvaro Herrera, Peter Eisentraut
Reviewed-by: Tristan Partin
Discussion: https://www.postgresql.org/message-id/ca8f37e1-a2c3-40e2-91f6-59c3d3652ad4@technowledgy.de
Backpatch: 16-, where meson support was added

meson.build

@@ -3270,15 +3270,17 @@ test_install_destdir = meson.build_root() / 'tmp_install/'
 if build_system != 'windows'
   # On unixoid systems this is trivial, we just prepend the destdir
   assert(dir_prefix.startswith('/')) # enforced by meson
-  test_install_location = '@0@@1@'.format(test_install_destdir, dir_prefix)
+  temp_install_bindir = '@0@@1@'.format(test_install_destdir, dir_prefix / dir_bin)
+  temp_install_libdir = '@0@@1@'.format(test_install_destdir, dir_prefix / dir_lib)
 else
   # drives, drive-relative paths, etc make this complicated on windows, call
   # into a copy of meson's logic for it
   command = [
     python, '-c',
     'import sys; from pathlib import PurePath; d1=sys.argv[1]; d2=sys.argv[2]; print(str(PurePath(d1, *PurePath(d2).parts[1:])))',
-    test_install_destdir, dir_prefix]
-  test_install_location = run_command(command, check: true).stdout().strip()
+    test_install_destdir]
+  temp_install_bindir = run_command(command, dir_prefix / dir_bin, check: true).stdout().strip()
+  temp_install_libdir = run_command(command, dir_prefix / dir_lib, check: true).stdout().strip()
 endif
 
 meson_install_args = meson_args + ['install'] + {
@@ -3315,7 +3317,6 @@ testport = 40000
 
 test_env = environment()
 
-temp_install_bindir = test_install_location / get_option('bindir')
 test_initdb_template = meson.build_root() / 'tmp_install' / 'initdb-template'
 test_env.set('PG_REGRESS', pg_regress.full_path())
 test_env.set('REGRESS_SHLIB', regress_module.full_path())
@@ -3330,7 +3331,7 @@ test_env.set('PG_TEST_EXTRA', get_option('PG_TEST_EXTRA'))
 # that works (everything but windows, basically). On windows everything
 # library-like gets installed into bindir, solving that issue.
 if library_path_var != ''
-  test_env.prepend(library_path_var, test_install_location / get_option('libdir'))
+  test_env.prepend(library_path_var, temp_install_libdir)
 endif