Exclude the build tree from the source package

The .gitignore-derived `build/` pattern does not actually prune the build
directory or its contents from the CPack source package, so an empty build/
(and, once docs were built in CI, build/doc-html) leaked into the zip. Ignore
the whole build tree in CPACK_SOURCE_IGNORE_FILES.

.github/workflows/release.yml

@@ -76,9 +76,9 @@ jobs:
 
       - name: Build source zip via CPack
         id: build
-        # Configure first so the doc target picks up doxygen/mkdocs, build the
-        # docs, then stage the generated site into the source tree so the CPack
-        # source package (which packages the source dir) includes doc-html/.
+        # Build the docs and stage the rendered site at the top level so the
+        # CPack source package includes doc-html/. The build/ tree itself is
+        # excluded by CPACK_SOURCE_IGNORE_FILES.
         run: |
           cmake -B build .
           cmake --build build --target doc

CMakeLists.txt

@@ -582,7 +582,9 @@ if (FMT_MASTER_PROJECT AND EXISTS ${gitignore})
     string(REPLACE "*" ".*" line "${line}")
     set(ignored_files ${ignored_files} "${line}$" "${line}/")
   endforeach ()
-  set(ignored_files ${ignored_files} /.git /build/doxyxml .vagrant)
+  # Exclude the whole build tree: the .gitignore-derived `build/` pattern does
+  # not actually prune the directory or its contents from the source package.
+  set(ignored_files ${ignored_files} /.git "/build($|/)" .vagrant)
 
   set(CPACK_SOURCE_GENERATOR ZIP)
   set(CPACK_SOURCE_IGNORE_FILES ${ignored_files})