bug#18440: libexec_LIBRARIES not allowed

[Dan Nicolaescu]

Trying to add a target with
libexec_LIBRARIES 
results in an error:

 error: 'libexecdir' is not a legitimate directory for 'LIBRARIES'

But it looks like having libraries in libexec is something already in
use by a few things.
On a Fedora 20 machine: 
$ find /libexec -name \*.so
/usr/libexec/p7zip/7z.so
/usr/libexec/coreutils/libstdbuf.so
/usr/libexec/sudo/sudo_noexec.so
/usr/libexec/sudo/sudoers.so
/usr/libexec/sudo/group_file.so
/usr/libexec/sudo/system_group.so
/usr/libexec/systemtap/libHelperSDT_amd64.so
/usr/libexec/gcc/x86_64-redhat-linux/4.8.3/liblto_plugin.so
/usr/libexec/deja-dup/tools/libduplicity.so

Shouldn't automake support installing libraries in libexec?