Non-default libproj

[anthonynorth]

Allow users to link to the non-default libproj, but still require libproj >= 6.3.1.

@jeroen's anticonf already has this capability (example in jeroen/curl) and is much simpler than autoconf, so perhaps it's worth adopting.

[mdsumner]

I never understood this properly before, but I'm having success with setting PKG_LIBS and PKG_CPPFLAGS and LD_LIBRARY_PATH env vars to point to a local build for a user, even with a distro-installed PROJ lib.

So,

export PKG_LIBS="-L/home/user/lib -lproj"
export PKG_CPPFLAGS="-I/home/user/include"
export LD_LIBRARY_PATH="/home/user/lib:$LD_LIBRARY_PATH"

is working for me in this gh version of PROJ and proj4.

I'll keep exploring, I haven't tested all cases or things I'm unsure of.