GitHub: Fix paths that cause builds

We should split build.yml into separate workflows for each OS, that way
we can have Windows builds triggered by changes to NTMakefiles, but not
Linux builds.

.github/workflows/build.yml

@@ -1,8 +1,51 @@
 name: Build
 
 on:
-    push: {paths: [src/**, .github/workflows/build.yml]}
-    pull_request: {paths: [src/**, .github/workflows/build.yml]}
+    push:
+      paths:
+         - '!docs/**'
+         - '!**.md'
+         - '!**.[1-9]'
+         - '**.[chly]'
+         - '**.hin'
+         - '**.in'
+         - '**.am'
+         - '**.m4'
+         - '**.ac'
+         - '**.pl'
+         - '**.py'
+         - '**.asn1'
+         - '**.opt'
+         - '**/NTMakefile'
+         - '**/COPYING'
+         - '**/INSTALL'
+         - '**/README*'
+         - '.github/workflows/build.yml'
+         - '!appveyor.yml'
+         - '!.travis.yml'
+
+    pull_request:
+      paths:
+         - '!docs/**'
+         - '!**.md'
+         - '!**.[1-9]'
+         - '**.[chly]'
+         - '**.hin'
+         - '**.in'
+         - '**.am'
+         - '**.m4'
+         - '**.ac'
+         - '**.pl'
+         - '**.py'
+         - '**.asn1'
+         - '**.opt'
+         - '**/NTMakefile'
+         - '**/COPYING'
+         - '**/INSTALL'
+         - '**/README*'
+         - '.github/workflows/build.yml'
+         - '!appveyor.yml'
+         - '!.travis.yml'
 
 jobs: