automake: allow only shared builds

Static and shared builds were possible in the good old days
of static makefiles. Currently the build system does not
distinguish nor does anything special when one requests a
static build.

Print a warning message for the packager that static builds
are not supported and continue building shared libs.

Currently only Debian and derivatives use static build, and
they use it for building a Xlib powered libGL. This patch
will only change the warning message they are seeing but
the binaries produced will be identical.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>