Add png16 and libpng16 to library names in libpng.jam (#514)

* Add png16 and libpng16 to library names in libpng.jam

On Windows, this allows to successfully find the library installed
using vcpkg which deploys the library named as libpng16.lib
and not png16.lib or png.lib.

* Add rationale behind listing multiple libpng names

src/tools/libpng.jam

@@ -16,15 +16,19 @@ import ac ;
 import errors ;
 import feature ;
 import "class" : new ;
-import targets ; 
+import targets ;
 import path ;
 import modules ;
 import indirect ;
 import property ;
 import property-set ;
 
 header = png.h ;
-names = png ;
+
+# On Windows, binary distributions of libpng and package managers
+# name the library differently (e.g. vcpkg installs libpng16.lib).
+# Listing popular names increases chances of successful look-up.
+names = libpng libpng16 png png16 ;
 
 sources =   png.c pngerror.c pngget.c pngmem.c pngpread.c pngread.c pngrio.c pngrtran.c pngrutil.c
             pngset.c pngtrans.c pngwio.c pngwrite.c pngwtran.c pngwutil.c ;