Check if C-sources are US-ASCII

Encoding of C-sources can not be determined, and non-ASCII code
are often warned by localized compilers.

.github/workflows/check_encoding.yml

@@ -0,0 +1,11 @@
+name: Check C-source encoding
+on: [push, pull_request]
+
+jobs:
+  check_encoding:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Check if C-sources are US-ASCII
+        run: |
+          ! grep -r -n '[^	 -~]' *.[chy] include internal win32/*.[ch]