Add a build-essential systemmodule

Submitted by Craig Scott Keogh @cskeogh

Link to original bug (#682761)

Description

As recommended by Colin in bug 681342 comment 3, JHBuild could support a special systemmodule "build-essential". "build-essential" will automatically become a dependency of every module - no need to explicitly add to every module.

Or how about a special systemmodule for each modtype? For example "autotools-essential" that includes automake, autoconf, libtool, etc. "autotools-essential" is only applicable to <autotools> modules. "cmake-essential" will have cmake. "waf-essential" will have waf, etc.

Or could do both?