Add build/pkg-install.sh.

[dot-asm]

Compile latest release tag and install it as pkg-config package.

[dot-asm]

To give it a shot execute following commands

mkdir -p /tmp/foo/include
mkdir -p /tmp/foo/lib/pkgconfig
curl -sSf https://raw.githubusercontent.com/dot-asm/blst/pkg-install.sh/build/pkg-install.sh | env PKG_CONFIG_PATH=/tmp/foo/lib/pkgconfig sh`

Examine /tmp/foo tree structure and cleanup as deemed appropriate.

[arrowd]

Bumped into this when porting libblst to FreeBSD. For now I just added my own .pc file to the downstream package, so it'd be great to have this PR merged in.