meson: explicitly add --libdir (#9504)

If we do not specify libdir explicitly, Meson chooses something like
lib/x86_64-linux-gnu, which causes problems when trying to find libraries
and pkg-config files.

lib/spack/spack/build_systems/meson.py

@@ -116,6 +116,11 @@ def _std_args(pkg):
 
         args = [
             '--prefix={0}'.format(pkg.prefix),
+            # If we do not specify libdir explicitly, Meson chooses something
+            # like lib/x86_64-linux-gnu, which causes problems when trying to
+            # find libraries and pkg-config files.
+            # See https://github.com/mesonbuild/meson/issues/2197
+            '--libdir={0}'.format(pkg.prefix.lib),
             '--buildtype={0}'.format(build_type),
             '--strip',
         ]
@@ -141,6 +146,7 @@ def meson_args(self):
         meson, except:
 
         * ``--prefix``
+        * ``--libdir``
         * ``--buildtype``
         * ``--strip``