media-session-uninstalled: build this script with the meson dirs filled in

This matches WirePlumber commit 52aaf9617

./builddir/media-session-uninstalled.sh can now be called without a -b
argument, it'll automatically set the full paths. This is useful when
building as a subproject.

meson has a limitation: the configure_file() object is just a file
wrapper object. Wrap this into a custom_target() whose return value has
a full_path() method. This way, pipewire can use
media_session_uninstalled.full_path() without having to build the
directory structure itself.

media-session-uninstalled.sh

@@ -2,7 +2,15 @@
 
 set -e
 
-SOURCEDIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
+# This is unset by meson
+# shellcheck disable=SC2157
+if [ -z "@MESON@" ]; then
+	SOURCEDIR="@MESON_SOURCE_ROOT@"
+	BUILDDIR="@MESON_BUILD_ROOT@"
+else
+	SOURCEDIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
+	BUILDDIR=${SOURCEDIR}/builddir
+fi
 
 while getopts ":b:v:" opt; do
 	case ${opt} in
@@ -24,11 +32,6 @@ while getopts ":b:v:" opt; do
 	esac
 done
 
-if [ -z "${BUILDDIR}" ]; then
-	BUILDDIR=${SOURCEDIR}/builddir
-	echo "Using default build directory: ${BUILDDIR}"
-fi
-
 if [ ! -d "${BUILDDIR}" ]; then
 	echo "Invalid build directory: ${BUILDDIR}"
 	exit 1

meson.build

@@ -121,9 +121,26 @@ if doxygen.found()
   subdir('doc')
 endif
 
-setenv = find_program('media-session-uninstalled.sh')
+conf_uninstalled = configuration_data()
+conf_uninstalled.set('MESON', '')
+conf_uninstalled.set('MESON_SOURCE_ROOT', meson.project_source_root())
+conf_uninstalled.set('MESON_BUILD_ROOT', meson.project_build_root())
+
+ms_uninstalled = configure_file(
+  input : 'media-session-uninstalled.sh',
+  output : 'media-session-uninstalled.sh.in',
+  configuration : conf_uninstalled,
+)
+
+media_session_uninstalled = custom_target('media-session-uninstalled',
+  output : 'media-session-uninstalled.sh',
+  input : ms_uninstalled,
+  build_by_default : true,
+  command : ['cp', '@INPUT@', '@OUTPUT@'],
+)
+
 run_target('media-session-uninstalled',
-           command : [setenv,
+           command : [media_session_uninstalled,
                       '-b@0@'.format(meson.project_build_root()),
                       '-v@0@'.format(media_session_version)]
 )