Fix autotools instalation with symbols

Closes #1447

When installing without symbols install-strip is used,
with symbols - install-exec.
Since install-exec installs only executables, we should use
simple install, to install other needed files also
(like files in /usr/shared) as install-strip does.

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@gmail.com>
Acked-by: Lionel Landwerlin <llandwerlin@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>