1. Modify project(PNG) to project(PNG C) to avoid search for C++ compiler
2. set(CMAKE_ALLOW_LOOSE_LOOP_CONSTRUCTS true) and fix end conditionals
from ugly else(FOO AND BAR) endif(FOO AND BAR) to else() endif()
3. Add missing include_directories() for ZLIB_INCLUDE_DIR
4. Created CMake options to allow configurable building of STATIC or SHARED
libpng, both default on.
5. Modified PNG_DEBUG to default to false?
6. Fix installation of static/shared libraries so DLL goes in prefix+/bin,
all others in prefix+/lib
