prevent pkg-config from finding unrelated system libraries

[vtjnash]

fix #12514

[tkelman]

Hm, ref #10010, it looks like this fails to find openssl in multilib compilation of 32 bit linux from 64 bit linux. That's better than a build/link failure, but may make testing https + libgit2 on 32 bit linux a little hard in the future. I can't seem to figure out how to get apt-get to install libssl-dev:i386 without uninstalling build-essential on ubuntu 14.04, so maybe we should go with this change anyway.

[vtjnash]

yeah, its likely this would break distributions, which may actually want to be using pkg-config (and would thus force them to unset / unexport this variable). failing to find openssl may actually be good, since if we're building generic linux binaries, then we really only want to find libs that will actually be on all target machines.

[tkelman]

Not quite. We bundle the openssl that is on the build machine. Ref #10763. No https on Linux without it, unfortunately. While there will almost always be a system version present on pretty much any Linux distribution, it's not guaranteed to be API or ABI compatible with the version we build libgit2 against on the buildbots. At least libgit2 can use native https on OS X and Windows.

@nalimilan do you need this pkg-config variable to be set while building julia? Will this trigger an rpmlint warning or anything?

[nalimilan]

Am I correct that it will break USE_SYSTEM_LIBGIT2=1 if not unsetting PKG_CONFIG_LIBDIR manually? Why not make it conditional then?

[tkelman]

How would this break USE_SYSTEM_LIBGIT2? Enabling that just means we don't build libgit2 and instead rely on ccall to find it somewhere on the path, pretty basic in this case.

[nalimilan]

Ah, right, I forgot that we don't actually link to libgit2. So should be fine with me.

[stevengj]

Does this work? http://stackoverflow.com/questions/23843106/how-to-set-environment-variable-in-makefile

[stevengj]

Oh, never mind, I see that that answer references the export command in make; I never knew about this one.

[vtjnash]

let's try this for a bit. if it turns out we need some .pc file, we can add rules to copy in the right ones from the right sysroot.

[nalimilan]

Actually this does break when using USE_SYSTEM_LIBGIT2=0 USE_SYSTEM_SSH2=1. In that case libgit2's CMake script doesn't detect libssh2 using pkg-config, so SSH is disabled.