maint: detect clang 2.9

maint: detect clang 2.9

In Fedora 15, with clang 2.8, 'scan-build env' shows:
CCC_ANALYZER_ANALYSIS=-analyzer-check-objc-mem -analyzer-check-security-syntactic -analyzer-check-dead-stores -analyzer-check-objc-unused-ivars -analyzer-check-objc-methodsigs

But in rawhide, with clang 2.9, the same variable is set but
empty, implying the default set of analysis.  We still want
sa_assert defined in that case, to stop clang from hitting
false positives.

* configure.ac (STATIC_ANALYSIS): Detect clang even when the set
of analyses is the default.