tests: Disable 113 generatorcustom on VS backends

Can only test this by checking the compiler id, but that's good enough.
Disabling so we can get #995 in which will help keep the VS backend in
a better state w.r.t. other PRs.

I've opened #1004 to track this in the meantime.

test cases/common/113 generatorcustom/meson.build

@@ -1,17 +1,21 @@
 project('generatorcustom', 'c')
 
-creator = find_program('gen.py')
-catter = find_program('catter.py')
+if meson.get_compiler('c').get_id() != 'msvc'
+  creator = find_program('gen.py')
+  catter = find_program('catter.py')
 
-gen = generator(creator,
-  output: '@BASENAME@.h',
-  arguments : ['@INPUT@', '@OUTPUT@'])
+  gen = generator(creator,
+    output: '@BASENAME@.h',
+    arguments : ['@INPUT@', '@OUTPUT@'])
 
-hs = gen.process('res1.txt', 'res2.txt')
+  hs = gen.process('res1.txt', 'res2.txt')
 
-allinone = custom_target('alltogether',
-    input : hs,
-    output : 'alltogether.h',
-    command : [catter, '@INPUT@', '@OUTPUT@'])
+  allinone = custom_target('alltogether',
+      input : hs,
+      output : 'alltogether.h',
+      command : [catter, '@INPUT@', '@OUTPUT@'])
 
-executable('proggie', 'main.c', allinone)
+  executable('proggie', 'main.c', allinone)
+else
+  error('MESON_SKIP_TEST: Skipping test on VS backend; see: https://github.com/mesonbuild/meson/issues/1004')
+endif