BUILD: Split configure.engines down to a single file per engine.

This is allows engines to be added dynamically to the build system.
They are placed into a folder in engines/ which must contain a file
named "configure.engine" to add the engine, which is pulled into the
top level configure script automatically. configure uses this to
generate all files required by the build system in the build directory.
DEVTOOLS: Factor out function to create directories in create_project.
DEVTOOLS: Adapt create_project for new configure.engine files.

I could not try any generated project files since I do not have access to
the IDEs.
BUILD: Add engines/plugins_table.h to git in the ideprojects target.
BUILD: Partial solution for parallel make issue.

This is due to the multiple outputs produced by the configure rule,
which cause multiple invocations of configure when make is run in
parallel. Various solutions are detailed in the Multiple-Outputs
section of the GNU automake manual which apply generally to makefiles.

This solution is a simpler one, but should solve the problem, though it
can fail on "mutilated" trees ie. where some of the configure outputs
are present, but not all... but this situation is not common, tends to
be due to an error in configure and should be recoverable by a
"make clean && ./configure" call.
BUILD: Add code to maintain ordering of engines in generated files.

This is mainly "cosmetic" to keep the SCUMM engine and subengines at
the top of the various files, but probably a good idea to prevent any
subtle regressions associated with changing the order.
DEVTOOLS: Make create_project sort SCUMM as first engine.

This makes create_project output consistent with configure output again.