Update pkg-config in manylinux build images to 0.29.2

[flavorjones]

Addresses concerns from #121

Note that:

• the original pkg-config is in /usr/bin
• this patch installs the new pkg-config into /usr/local/bin
• and /usr/local/bin is ahead of /usr/bin in $PATH so the new one should be used by default

If this works, then I'll add some documentation around it.

[flavorjones]

cc @mudge

[mudge]

Thanks, @flavorjones. What’s the best way for me to test this out?

[flavorjones]

Comparing re2's x86_64-linux build on my laptop between the 1.5.0 image and an image generated from this PR's branch ("1.5.1.dev"):

• 1.5.0 takes 14m23.800s
• 1.5.1.dev takes 1m34.937s (!)

@mudge Objections to me merging this and shipping 1.5.1?

[mudge]

None whatsoever!

[flavorjones]

@mudge Here are the steps I took:

1. on dev machine, clone this branch, run gem build, and run gem install path/to/rake-compiler-dock-1.5.1.dev.gem
2. in your re2 working directory, update re2.gemspec to depend on 1.5.1.dev
3. bundle update and bundle package so that your r-c-d builds can access the gem from vendor/cache

That was it.