Preserve -L -l pairings fetched from external deps

While adding link args for external deps, sometimes different
libraries come from different prefixes, and an older version of the
same library might be present in other prefixes and we don't want to
accidentally pick that up.

For example:

/usr/local/lib/libglib-2.0.so
/usr/local/lib/pkgconfig/glib-2.0.pc
/usr/local/lib/libz.so
/usr/local/lib/pkgconfig/zlib.pc
/home/mesonuser/.local/lib/libglib-2.0.so
/home/mesonuser/.local/lib/pkgconfig/glib-2.0.pc

PKG_CONFIG_PATH="/home/mesonuser/.local/lib/pkgconfig/:/usr/local/lib/pkgconfig/"

If a target uses `dependencies : [glib_dep, zlib_dep]`, it will end up
using /usr/local/lib/libglib-2.0.so instead of
/home/mesonuser/.local/lib/libglib-2.0.so despite using the pkg-config
file in /home/mesonuser/.local/lib/pkgconfig because we reorder the -L
flag and separate it from the -l flag.

With this change, external link arguments will be added to the
compiler list without de-dup or reordering.

Closes https://github.com/mesonbuild/meson/issues/1718

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