Merge build-utils-meson/{diagnostics,threads} into build-utils-meson/common

This reduces the amount of boilerplate. More importantly, it provides
a place to add compiler flags (such as -O3) without having to add it
to every subproject (and the risk of forgetting to include it).

build-utils-meson/common/meson.build

@@ -0,0 +1,18 @@
+# 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',
+)