mesa: Optionally build a dricore support library (v3)

mesa: Optionally build a dricore support library (v3)

This an adds --enable-shared-dricore option to configure.  When enabled,
DRI modules will link against a shared copy of the common mesa routines
rather than statically linking these.

This saves about 30MB on disc with a full complement of classic DRI
drivers.

v2: Only enable with a gcc-compatible compiler that handles rpath
    Handle DRI_CFLAGS without filter-out magic
    Build shared libraries with the full mklib voodoo
    Fix typos
v3: Resolve conflicts with talloc removal patches

Signed-off-by: Christopher James Halse Rogers <christopher.halse.rogers@canonical.com>