Only enable implicit-fallthrough for gcc
Only enable implicit-fallthrough for gcc

It turns out that clang passes the configure test for
implicit-fallthrough=3, but then fails compilation.

Sigh.

Enabling it only for gcc (that supports it) is good enough to
keep the codebase clean.

icing/mod_md,icing/mod_md,icing/mod_md,icing/mod_md,icing/mod_md