CA-119833: snapwatchd fails to link on some systems

snapwatchd fails to link a shared .so with -shared:

  /usr/bin/ld: xslib.o: relocation R_X86_64_32 against `.rodata.str1.1'
  can not be used when making a shared object
  xslib.o: could not read symbols: Bad value

We need to explicitly compile our objects as position-independent
code with -fPIC

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Acked-by: Germano Percossi <germano.percossi@citrix.com>

GitHub: closes #49 on xapi-project/sm
[CA-118876] & [SCTX-1536]: Always use the back-up VHD footer.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Keith Petley <keith.petley@citrix.com>

GitHub: closes #51 on xapi-project/sm