Do not store compiled objects outside the project tree.

Define SOURCES instead of OBJS in Makefiles. ...

OBJS is still deduced from SOURCES, but this change gives better control
over where the object files get actually written.

Use make variables $@ and $^ where appropriate.

Ensure `make clean` is idempotent. ...

Prior to this, `make clean` would fail if the project was not already
fully built, and a second invokation would always fail.

Examples: Makefile leave CXX alone, instead leave commented out options. (nb: codeship doesn't seem have clang++)

Examples: Use Clang in old Makefile.

Examples: Synced Makefile comments. Added build/ folder into git ignore list/

Examples: GLFW+GL2: Renamed imgui_impl_glfw.* to imgui_impl_glfw_gl2.* for consistency and to emphasis on GL2-ness.

add SDL GL3 makefile

fix gl2 sample too

Examples: Speculative fix for OSX Makefile to make Travis happy (re #812)

Examples: Renaming opengl_example/ to opengl2_example/ for clarity (1/2 because git)