build(meson): warn/fail on 32-bit machines (#2181)

On 32-bit Windows, meson setup fails with an unclear error:

    meson.build:25:16: ERROR: Could not get define 'CPPHTTPLIB_VERSION'

The actual problem is that httplib.h #errors out.

Have the Meson logic explicitly check for a 32-bit host and warn or error,
matching the check in httplib.h.  Phrase the Windows error in a way that
triggers WrapDB CI's unsupported architecture check.

meson.build

@@ -18,6 +18,14 @@ project(
 
 cxx = meson.get_compiler('cpp')
 
+if cxx.sizeof('void *') != 8
+  if host_machine.system() == 'windows'
+    error('unsupported architecture: cpp-httplib doesn\'t support 32-bit Windows. Please use a 64-bit compiler.')
+  else
+    warning('cpp-httplib doesn\'t support 32-bit platforms. Please use a 64-bit compiler.')
+  endif
+endif
+
 # Check just in case downstream decides to edit the source
 # and add a project version
 version = meson.project_version()