In configure, --with-libpng doesn't seem to take effect

[csun-mw]

Hi,
I'm following the instructions herehttps://pngquant.org/install.html
 to build from the source downloaded from Github. When using the options --prefix=CUSTOM_PATH --with-libpng=MY_LIBPNG_DIR with the configure file, though, those options don't seem to take effect.

• I can build pngquant on a Debian machine properly, but it outputs the binary in the source directory, not the one I specified with --prefix;
• When building it on a Mac, which has libpng 1.6.36 installed on a custom folder (not as a subfolder of pngquant source), though I typed in the right libpng path for --with-libpng, the build would run into the warning and error like the following, and fail: 

warning: pkg-config is not installed, can't search for libraries. Please install pkg-config.    libpng: error ... not found (try: brew install libpng)

Any help will be much appreciated.