OS_LIBS for libraries that are not part of the gecko tree, EXTRA_LIBS for
libraries, such as NSPR, that are in the tree, but are not handled by
moz.build just yet. Those EXTRA_LIBS may also come from a system library.
However, in cases where the expanded variables are always empty for the
in-tree case, OS_LIBS is used (as for, e.g. MOZ_ZLIB_LIBS). OS_LDFLAGS is
used exclusively for non-library linker flags.
Always pass EXTRA_LIBS before OS_LIBS on linker command lines.
Forbid EXTRA_DSO_LDOPTS, SHARED_LIBRARY_LIBS and LIBS in Makefiles.
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