[PATCH] meson: update doc logic for Windows

[Vipin Varghese]

Support for shell scripts doxy-html-custom, generate_doxygen and
generate_examples are absent. The current patch address the same by
disabling document build notifying the user.

Steps to reproduce the error:
 - Install dependencies doxygen & sphinix build on Windwos server 2019.
 - Build DPDK with option enable_docs=true for API or User Guide.

This produces error
```
FAILED: doc/api/examples.dox
sh -e dpdk/doc/api/generate_examples.sh dpdk/examples doc/api/examples.dox
```

Signed-off-by: Vipin Varghese <vipin.varghese at amd.com>
---
 doc/meson.build | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/doc/meson.build b/doc/meson.build
index 6f74706aa2..d7c35a12f2 100644
--- a/doc/meson.build
+++ b/doc/meson.build
@@ -3,6 +3,14 @@
 
 doc_targets = []
 doc_target_names = []
+
+if is_windows
+    if get_option('enable_docs')
+        message('Docs target not supported on windows')
+    endif
+    subdir_done()
+endif
+
 subdir('api')
 subdir('guides')
 
-- 
2.25.1