configure: Always build with -fno-common

MacOSX doesn't pull .o files from .a archives if the symbol that it
requires is one which the .o file defines as a common symbol.
(Common symbols are those declared without "extern"; the linker
will merge together common symbols with the same name, so
redeclaring the same variable in two compilation units results in
them referring to the same symbol rather than a compilation error).

This MacOSX difference from traditional linker behaviour means that
"make check" produces link errors:

Undefined symbols for architecture x86_64:
  "_cur_mon", referenced from:
      _error_vprintf in libqemuutil.a(qemu-error.o)
      _error_printf in libqemuutil.a(qemu-error.o)
      _error_printf_unless_qmp in libqemuutil.a(qemu-error.o)
      _error_print_loc in libqemuutil.a(qemu-error.o)
      _error_report in libqemuutil.a(qemu-error.o)
ld: symbol(s) not found for architecture x86_64

in this case because "cur_mon" is a common symbol in
libqemustub.a(mon-set-error.o).

In QEMU we don't make any use at all of the common symbol
functionality, so we can avoid this problem entirely simply
by compiling with -fno-common. Enable this option for all
builds, not just MacOSX, so that if we ever inadvertently
introduce multiple definitions of some variable that will
be immediately spotted as a build error rather than only
breaking the MacOSX build.

Suggested-by:  Markus Armbruster <armbru@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Message-id: 1393451610-24617-1-git-send-email-peter.maydell@linaro.org
