plugin/c_src: compile C++ source

Add compile .cc,.cpp,.C files as C++.
Compile each source file to .o.
Link all .o object to .so.
Delete .o on clean target.
plugin/c_src: remove C_SRC_OPTS

We can just LDFLAGS += and whatnot.
Merge branch 'C++_source' of git://github.com/nevar/erlang.mk
Update README for C/C++ compiler
Add missing variable to the README
add escript plugin
Merge branch 'add-escript' of git://github.com/dch/erlang.mk
plugins/c_src.mk: use non-regex file extention detection

* The `-regex` option of `find` command to determine `SOURCES` in
  plugins/c_src.mk` is not portable; does not work on OS X.
  Rewrite with a portable form of using `find` native `-name`
  command option with `\(`, `-o`, and `\)`.

* Support file extentions of `*.c`, `*.C`, `*.cc`, and `*.cpp`.
plugins/c_src.mk: update CFLAGS/CXXFLAGS/LDFLAGS for OS X

* Add the following flags to LDFLAGS needed at linkage for OS X,
  and remove the following flags from CFLAGS and CPPFLAGS:
  `-arch x86_64 -flat_namespace -undefined suppress`
Merge branch 'jj1bdx-c_src-find-fix' of git://github.com/jj1bdx/erlang.mk
Group clean-app commands

Smaller non-verbose output.
Remove +warn_export_all from the enabled warnings by default

People who don't want export_all simply don't use it. The
warning is annoying for those edge cases where it's actually
useful, as it can't be disabled on a per-file basis.