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A general description is here.
When building R as a shared library, the ‘-L${R_HOME}/lib${R_ARCH}’ flag is placed earlier in the link commands used during installation and when packages are installed: this help ensure that the current build has priority if an R shared library has already been installed by e.g. ‘install-libR’ in a library mentioned in ‘LDFLAGS’ (and not in ‘your system's library directory’ as documented). (Wish of PR#15790.)
There is a new target ‘make uninstall-libR’ to remove an installed shared/static ‘libR’.
‘make install-libR’ now works if a sub-architecture is used, although the user will need to specify ‘libdir’ differently for different sub-architectures.