Need to have type 'custom' handled correctly as data-delivering target

[ethouris]

The custom type target (for a case of providing installable data) must have the following characteristics:

• Target name is always phony. The exact files produced by this target are mentioned under the output key
• The files required by the output to produce are listed under sources key
• This type must use the data keyed at usein so that it's pinned into either install-TARGET-runtime or install-TARGET-devel make targets
• When the target is a dependency of a library, then it must be produced before building the library. This might be also non-installable in this case
• When (REVERSE TO THE ONE ABOVE!) the target is dependent on a library, then it must have installation rule defined - the install directory as well as the usein key. This only means that the library must be built prior to running the actions for data target.
• The make's install target for this target must be built, both install-TARGET-devel and install-TARGET-runtime bound together with install-TARGET. Which of the first two will be placed the installation action to, depends on the usein key.

Overall fix:

• The general install-devel and install-runtime targets must be defined, but not necessarily as dependencies of install (this target should integrate all installation targets, regardless of affiliations). The install-runtime must select all installation targets that are (also indirectly) derived from all target and have form of runtime installation - that is, it is so for programs, runtime-usein data, and libraries. The install-devel must embrace everything else that has installation target and does not match the "install-runtime" category.