test: Add a make install target for the tests

This makes it more convenient to compile them alongside SDL, install
them in an optional package and use them as smoke-tests or diagnostic
tools. The default installation directory is taken from GNOME's
installed-tests, which seems as good a convention as any other:
https://wiki.gnome.org/Initiatives/GnomeGoals/InstalledTests

Signed-off-by: Simon McVittie <smcv@collabora.com>

test/configure.ac

@@ -99,6 +99,15 @@ AC_SUBST(ISWINDOWS)
 AC_SUBST(ISUNIX)
 AC_SUBST(ISOS2)
 
+AC_SUBST([prefix])
+AC_SUBST([exec_prefix])
+AC_SUBST([bindir])
+AC_SUBST([libdir])
+AC_SUBST([libexecdir])
+AC_SUBST([includedir])
+AC_SUBST([datarootdir])
+AC_SUBST([datadir])
+
 dnl Check for SDL
 SDL_VERSION=2.0.18
 AM_PATH_SDL2($SDL_VERSION,