Change to C99 mode and disable a warning incompatible with C99

Summary:
Subject says it all.  -Wdeclaration-after-statement isn't
compatible with C99 (since it's a warning about a feature now
allowed in C99), so it needs to be removed.

Test Plan:
A change that didn't like -Wdeclaration-after-statement now
builds.  Also confirmed with ps that -std=c99 is on the gcc
command lines.

Reviewers: steaphan

Reviewed By: steaphan