configure script doesn't fall back to gcc if a bad version of clang is available

[brson]

The configure script prefers to use clang over gcc, but if it finds the wrong version of clang it just fails. It should instead print a warning and fall back to gcc.

[sayrer]

yes please! this is the default on OS X these days.

[msullivan]

This is related to #635.

[brson]

Fixed by pcwalton

[wting]

This issue should be reopened as current behavior when clang fails is to exit the build process (as tested with v0.5, v0.6 (master|incoming)).