meson: Fix installation path computation

We have the long-standing logic to append "postgresql" to some
installation paths if it does not already contain "pgsql" or
"postgres".  The existing meson implementation of that only considered
the subdirectory under the prefix, not the prefix itself.  Fix that,
so that it now works the same way as the implementation in
Makefile.global.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/a6a6de12-f705-2b33-2fd9-9743277deb08@enterprisedb.com

meson.build

@@ -465,22 +465,24 @@ pkg = 'postgresql'
 
 dir_prefix = get_option('prefix')
 
+dir_prefix_contains_pg = (dir_prefix.contains('pgsql') or dir_prefix.contains('postgres'))
+
 dir_bin = get_option('bindir')
 
 dir_data = get_option('datadir')
-if not (dir_data.contains('pgsql') or dir_data.contains('postgres'))
+if not (dir_prefix_contains_pg or dir_data.contains('pgsql') or dir_data.contains('postgres'))
   dir_data = dir_data / pkg
 endif
 
 dir_sysconf = get_option('sysconfdir')
-if not (dir_sysconf.contains('pgsql') or dir_sysconf.contains('postgres'))
+if not (dir_prefix_contains_pg or dir_sysconf.contains('pgsql') or dir_sysconf.contains('postgres'))
   dir_sysconf = dir_sysconf / pkg
 endif
 
 dir_lib = get_option('libdir')
 
 dir_lib_pkg = dir_lib
-if not (dir_lib_pkg.contains('pgsql') or dir_lib_pkg.contains('postgres'))
+if not (dir_prefix_contains_pg or dir_lib_pkg.contains('pgsql') or dir_lib_pkg.contains('postgres'))
   dir_lib_pkg = dir_lib_pkg / pkg
 endif
 
@@ -490,7 +492,7 @@ dir_include = get_option('includedir')
 
 dir_include_pkg = dir_include
 dir_include_pkg_rel = ''
-if not (dir_include_pkg.contains('pgsql') or dir_include_pkg.contains('postgres'))
+if not (dir_prefix_contains_pg or dir_include_pkg.contains('pgsql') or dir_include_pkg.contains('postgres'))
   dir_include_pkg = dir_include_pkg / pkg
   dir_include_pkg_rel = pkg
 endif