rpm: generate .spec based on CMake options and detection

Conditionally enable some FSALs, disable others.

1. rpmbuild accepts these options (gpfs as example):
   --with gpfs
   --without gpfs
2. CMake enables FSALs by detecting available support on the build system.

Build the FSAL when "--with <fsal>" or CMake detected support.
Override CMake's decision by passing "--without <fsal>" to rpmbuild.

Signed-off-by: Niels de Vos <ndevos@redhat.com>
CMake: add a 'make srpm' target

It is very convenient to create a src.rpm and have it rebuild for
different distributions. A 'make srpm' target can then be used to create
automated builds, for example with help from 'mock' or Fedora COPR.

Signed-off-by: Niels de Vos <ndevos@redhat.com>