nxagent/Imakefile: remove ../../exports/lib

nxagent links system libraries that link against libX11. Unfortunately,
nxagent (and libXcompshad) require a modified libX11 version,
nicknamed libNX_X11, for proper functioning. Fortunately, this one can
act as a drop-in replacement for the system libX11. So we’ll hack our
way out: add a DT_NEEDED entry for libX11 by linking against the
system library when building nxagent and link the other system
libraries later and set DT_RUNPATH to a special directory containing
symlinks from libNX_X11 to libX11.

This tricks the loader into pulling in the "fake" libX11 version
without checking its SONAME and thus satisfying the DT_NEEDED entry
early on - specifically also for the system libraries.

Ex.:
readelf -a /usr/bin/nxagent | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libX11.so.6]
...
 0x0000000000000001 (NEEDED)             Shared library: [libNX_X11.so.6]
...

If, however, you run the build a second time, it will find the newly
created libX11 links in ../../exports/lib (used as an additional
library search path) instead of the system libX11. The results in a
binary looking like this:

 0x0000000000000001 (NEEDED)             Shared library: [libNX_X11.so.6]
...

With such a setup, the whole magic falls apart and system libraries
will suddenly pull in the actual system libX11 file.

We initially believed a compiler call such as:

${CC} [--unrelated-options ...] [-Lunrelated_library_path ...] -lX11 [more
       options like -l and -L] -L../../exports/lib -lother_libraries

to do "the right thing" and link against the system libX11 (or, at
worst, a libX11 found in the library directory search list preceding the
link call) and use ../../exports/lib only for later linking operations,
since the order of options matters.

However, this turned out to be blatantly wrong: while the order of -L
arguments does matter for building the search path, the whole search
path including elements from *ALL* -L arguments (and the system paths)
will always be used when linking libraries.

Hence, (counter-intuitively to us) both these calls will be equivalent:

${CC} [--unrelated-options ...] -la -Loverride_liba -lb
      [-Lunrelated_library_path ...] -lc

${CC} [--unrelated-options ...] -Loverride_liba [-Lunrelated_library_path
       ...] -la -lb -lc

By removing LDPRELIBS from NXAGENTSYSLIBS, ../../exports/lib is no
longer used during building/linking and the binaries are built
reproducibly. We never intended to use this directory at link time
anyway.

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