Add a setup hook for fixing dylib install names on Darwin

Install names need to be absolute paths, otherwise programs that link
against the dylib won't work without setting $DYLD_LIBRARY_PATH.  Most
packages do this correctly, but some (like Boost and ICU) do not.
This setup hook absolutizes all install names.

pkgs/build-support/setup-hooks/fix-darwin-dylib-names.sh

@@ -0,0 +1,35 @@
+# On Mac OS X, binaries refer to dynamic library dependencies using
+# either relative paths (e.g. "libicudata.dylib", searched relative to
+# $DYLD_LIBRARY_PATH) or absolute paths
+# (e.g. "/nix/store/.../lib/libicudata.dylib").  In Nix, the latter is
+# preferred since it allows programs to just work.  When linking
+# against a library (e.g. "-licudata"), the linker uses the install
+# name embedded in the dylib (which can be shown using "otool -D").
+# Most packages create dylibs with absolute install names, but some do
+# not.  This setup hook fixes dylibs by setting their install names to
+# their absolute path (using "install_name_tool -id").  It also
+# rewrites references in other dylibs to absolute paths.
+
+fixDarwinDylibNames() {
+    local flags=()
+    local old_id
+
+    for fn in "$@"; do
+        flags+=(-change "$(basename "$fn")" "$fn")
+    done
+
+    for fn in "$@"; do
+        if [ -L "$fn" ]; then continue; fi
+        echo "$fn: fixing dylib"
+        install_name_tool -id "$fn" "${flags[@]}" "$fn"
+    done
+}
+
+fixDarwinDylibNamesIn() {
+    local dir="$1"
+    fixDarwinDylibNames $(find "$dir" -name "*.dylib")
+}
+
+postFixup() {
+    fixDarwinDylibNamesIn "$prefix"
+}