Fix cmake to be able to search custom libpng include path

[kogaki]

This PR is similar approach as #85 .

I bumped into the problem discussed in #79 . With this fix and some cmake option, I could finally build OpenToonz successfully.

Here's my cmake options

cd toonz/build
CMAKE_PREFIX_PATH=~/Qt5.5.1/5.5/clang_64 cmake \
  -DTIFF_INCLUDE_DIR=../../thirdparty/tiff-4.0.3/libtiff/ \
  -DPNG_PNG_INCLUDE_DIR=../../thirdparty/libpng-1.6.21/  \
  ../sources
make

[kogaki]

Since -DPNG_PNG_INCLUDE_DIR looks weird, CMakeLists.txt needs more fix.
This commit is just a workaround.

[tomosu]

I confirmed it works on my environmnet.