Normally, the build system creates libraries as mere side effects of
building targets. There is no way to explicitly trigger the build of
libraries only. However, in some circumstances (for example for testing
the thorough build of all libraries) a mechanism for explicitly building
libraries would be convenient. This patch implements this feature. It
consists of two changes.
The new pseudo target at 'base/src/lib/target.mk' gathers all libraries
that are available in all repositories specified for the build directory
and makes its target depend on them. This way, by building 'lib', all
libraries would be traversed. However, in the (likely) situation that
those libraries include one or more invalid libraries (libraries with
unsatisfied build requirements), the build system would skip the target.
Hence, the second change introduces a new condition 'FORCE_BUILD_LIBS'
to the build system. By setting this variable to 'yes' in the 'target.mk'
file, we let the build system to traverse library dependencies for
all valid libraries regardless of the presence of any invalid library.
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