Only move shared libraries to /lib if they exists

Only move shared libraries to /lib if they exists

In several Makemodule.am, there is a install-exec-hook-<library>
target whose role is to move the shared library from /usr/lib to /lib,
while keeping a symbolic link /usr/lib/libuuid.so ->
../../lib/<library>.so.<version>.

However, when util-linux is built with --enable-static
--disable-shared (as is needed on noMMU platforms that don't support
shared libraries), no <library>.so is built, but the
install-exec-hook-libuuid creates an invalid /usr/lib/<library>.so
symbolic link, pointing to ../../lib (yes, the directory). This causes
troubles later one when other libraries/programs are compiled with
-l<library>, as gcc thinks a shared library is available because
there's a file named /usr/lib/<library>.so.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>