Add meson target to update single include (#86)

- You can now update the single include file in a meson build by using
the target 'amalg'.
- The meson build also now builds the single include test suite.
- cmake build updated to use the new script to update the single
include.

scripts/update_single_include.sh

@@ -0,0 +1,8 @@
+#!/usr/bin/env sh
+
+if [ "${MESON_SOURCE_ROOT}" != "" ]
+then
+    cd ${MESON_SOURCE_ROOT}
+fi
+
+python3 amalgamate/amalgamate.py -c amalgamate/config.json -s include -v yes