doc: add spa to doxygen output

This requires a helper script: doxygen doesn't differ between static methods
and static inline methods. EXTRACT_STATIC defines whether it parses *any*
static method but we're currently using all C files as input files as well. We
cannot convince doxygen to just parse static inline functions in header files
so for SPA we hack around this: meson passes the spa headers to a shell script
with simply copies those changed to `/* static */ inline void (foo)` and doxygen
then runs on those header files.

The result: we get all spa functions added to your doxygen output at the cost
of a few sed calls.

doc/strip-static.sh

@@ -0,0 +1,18 @@
+#!/bin/bash
+#
+# Replicates the source tree at $SOURCE to $DEST with any static removed
+# from the file. This allows Doxygen to parse the file and document static
+# inline functions.
+
+SOURCE="$1"
+DEST="$2"
+test -n "$SOURCE" || (echo "Source argument is missing" && exit 1)
+test -n "$DEST" || (echo "Dest argument is missing" && exit 1)
+
+echo "Reading from $SOURCE"
+echo "Copying to $DEST"
+
+mkdir -p "$DEST"
+cp -rf "$SOURCE"/* "$DEST/"
+shopt -s globstar  # proper recursive globbing
+sed -i 's|^static|/* \0 */|' "$DEST"/**/*.h