Make configure script compatible with Homebrew
Make configure script compatible with Homebrew

The configure script seems to be compatible with Macports which installs
its stuff into /opt/local.

However, Homebrew doesn't have the same behavior. It installs its stuff
into a Cellar folder, then, links it into /usr/local/{include,lib,bin}.

However, for example, the png detection uses -Iprefix/include (line
1225), but this include path is not used afterwards.

So, if you have installed libpng with Homebrew, configure detects a
"system" png, but fails to compile since $prefix/include isn't used as
include path at compile time.

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