find_system_library: fix var cleanup when library not found

The "not found" path is attempting to clear SOMELIB_CFLAGS and
SOMELIB_LIBS by resetting them in AC_SUBST(). However, the second arg to
AC_SUBST is expanded in autoconf with m4_ifvaln([$2], [[$1]=$2]),
which is defined as "if the first arg is non-empty". The m4 "empty"
construction is [], therefore, the existing AC_SUBST calls never modify
the variables at all.

The effect of this is that leftovers from the library test can leak out.
At least, if a library header is found in the first stage, but the
library itself is not, -lsomelib is added to SOMELIB_LIBS and further
tests done. If that library is not found, SOMELIB_LIBS will not be
cleared.

For most of our library tests this hasn't been a problem, as they're
either always found properly via pkg-config or set directly, or the
calling test immediately aborts configure. For an optional dependency
however, an apparent "partial" result where the header is found but no
corresponding library causes link errors later.

I think a complete fix should probably not be setting SOMELIB_xxx until
the final result is known, but for now, adjusting the AC_SUBST calls to
explictly set the empty shell string (which is not "empty" to m4) at
least restores the intent.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #16140