I am following the GAGE workflow (here), but cannot install GAGE and its dependencies ('png' and  'KEGGREST').

During installation, R tests if installed packages can be loaded which leads to this error message:

Error in dyn.load(file, DLLpath = DLLpath, ...) :
  unable to load shared object '/home/usr/R/x86_64-pc-linux-gnu-library/3.2/png/libs/png.so':
  libpng16.so.16: cannot open shared object file: No such file or directory
Error: loading failed
Execution halted
ERROR: loading failed

So I installed libpng-1.6.19 from source;

$ ./configure --prefix=/usr/bin/libpng/
# /usr/bin/ being a $PATH directory
$ make
$ make install

Now I have '/usr/bin/libpng/lib/libpng16.so.16', but still impossible to install 'gage', 'png' or 'KEGGREST' (same error message).

(1) Anyone could tell me what I am doing wrong?!!!

(2) Do the binaries 'libpng16-config', 'png-fix-itxt' and 'pngfix' (located under /usr/bin/libpng/bin') need to be moved somewhere?

Note: I also tried to move 'libpng16.so.16' (which is a link) to my $PATH directory '/usr/bin' but still the same.