fixes to the static compilation case for sdl

After the next commit, pkg-config could be used for the shared library
configuration case and sdl-config for static libraries.  So I prepare
the test here by doing two changes:

at the same time I remove useless backslashes from the invocation of
grep;

1) fixing a typo ($sd_cflags).  The typo has been there since commit
1ac88f2 (remove sdl_static. Just do the right thing if static is yes,
2009-07-27).

2) fixing an erroneous "test `... | grep > /dev/null`" idiom that would
never succeed since grep's output would be empty;

3) checking the status code after executing sdl-config --static --libs;
this is needed for the next patch only.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
