meson: Mostly assume clang-cl is MSVC

We need to enable building the dirent and gnulib sources for clang-cl,
as we are still using the Microsoft-style headers and lib's and CRT.
We need to also do this for the following, for similar reasoning:

-Symbol export (via __declspec(dllexport))
-Dependency discovery without pkg-config files
-long long and ssize_t detection

We do, however, enable the autoptr tests for clang-cl builds.  Note that
at this point real MSVC builds are still better supported than clang-cl
builds, and it will likely remain so for at least the near future,
alhtough real MSVC builds of the GTK stack are consumable and are usable
by clang-cl.

gio/tests/meson.build

@@ -97,7 +97,7 @@ test_env.set('GIO_LAUNCH_DESKTOP', meson.build_root() + '/gio/gio-launch-desktop
 # 1.2.14 required for dbus_message_set_serial
 dbus1_dep = dependency('dbus-1', required : false, version : '>= 1.2.14')
 if not dbus1_dep.found()
-  if cc.get_id() == 'msvc'
+  if cc.get_id() == 'msvc' or cc.get_id() == 'clang-cl'
     # MSVC: Search for the DBus library by the configuration, which corresponds
     # to the output of CMake builds of DBus.  Note that debugoptimized
     # is really a Release build with .PDB files.
@@ -382,7 +382,7 @@ test_extra_programs += {
   },
 }
 
-if cc.get_id() != 'msvc'
+if cc.get_id() != 'msvc' and cc.get_id() != 'clang-cl'
   test_extra_programs += {
     # These three are manual-run tests because they need a session bus but don't bring one up themselves
     # FIXME: these build but don't seem to work!