When an older version of the library being built is installed in the
same prefix as external dependencies, we have to be careful to construct
the linker or compiler command line. If a -L flag from external
dependencoes comes before a -L flag pointing to builddir, it is possible
for the linker to load older libraries from the installation prefix
instead of the newly built ones, which is likely to cause undefined
reference error.
Since the order of dependencies is not significant, we cannot expect
internal dependencies to appear before external dependencies when
recursively iterating the list of dependencies. To make it harder to
make mistakes, linker flags come from internal and external
dependencies are now stored in different order sets. Code using
_get_dependencies_flags are expected to follow the order when
constructing linker command line:
1. Internal linker flags
2. LDFLAGS set by users
3. External linker flags
It is similar to what automake and libtool do for autotools projects.
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