rename: build-utils-meson -> nix-meson-build-support

Fix a footgun. In my case, I had a couple of build ("output")
directories sitting around.

    rm -rf build-*

Was confused for a bit why a meson.build file was missing.

Probably also helps with autocompletion.

I tried meson-build-support first, but I had to add something like
a nix- prefix, in order to make meson happy. They've reserved the
meson- prefix.

nix-meson-build-support/common/meson.build

@@ -0,0 +1,22 @@
+# This is only conditional to work around
+# https://github.com/mesonbuild/meson/issues/13293. It should be
+# unconditional.
+if not (host_machine.system() == 'windows' and cxx.get_id() == 'gcc')
+  deps_private += dependency('threads')
+endif
+
+add_project_arguments(
+  '-Wdeprecated-copy',
+  '-Werror=suggest-override',
+  '-Werror=switch',
+  '-Werror=switch-enum',
+  '-Werror=unused-result',
+  '-Wignored-qualifiers',
+  '-Wimplicit-fallthrough',
+  '-Wno-deprecated-declarations',
+  language : 'cpp',
+)
+
+if get_option('buildtype') not in ['debug']
+  add_project_arguments('-O3', language : 'cpp')
+endif