tests/common/93: Fix cross building when no exe_wrapper is available

By building the generator for the build machine always, and only for the
host machine if an exe_wrapper is available. This makes sense to me as
generally you are going to build the generator for the build machine,
not the host machine, but testing on the host machine makes sense too.

test cases/common/93 selfbuilt custom/meson.build

@@ -17,7 +17,7 @@ test('maintest', main)
 
 lib = library('libtool', 'tool.cpp')
 
-checkarg = executable('checkarg', 'checkarg.cpp')
+checkarg = executable('checkarg', 'checkarg.cpp', native : true)
 
 ctlib = custom_target('ctlib',
   output : 'ctlib.out',
@@ -25,3 +25,15 @@ ctlib = custom_target('ctlib',
   command : [checkarg, lib],
   build_by_default : true,
 )
+
+if meson.is_cross_build() and meson.has_exe_wrapper()
+  checkarg_host = executable('checkarg_host', 'checkarg.cpp')
+
+  ctlib_host = custom_target(
+    'ctlib_host',
+    output : 'ctlib.host.out',
+    capture : true,
+    command : [checkarg_host, lib],
+    build_by_default : true,
+  )
+endif