I'm not sure how that'd be possible. It uses blocks (^) so it requires blocks with clang, no?

I've changed my macro to yours, and compiled with clang (3.9)

  CC=clang-3.9 make clean indent depend all

  clang-3.9 -c -O3 -D_GNU_SOURCE -Wall -Werror -Wframe-larger-than=131072 -Wno-format-truncation -I. -std=c11 -I/usr/local/include -Wextra -Wno-override-init -funroll-loops -D_FILE_OFFSET_BITS=64 -D_HF_LINUX_INTEL_PT_LIB -Wno-initializer-overrides -Wno-unknown-warning-option -Wno-gnu-empty-initializer -Wno-format-pedantic -Wno-gnu-statement-expression -D_HF_ARCH_LINUX  -o cmdline.o cmdline.c
  In file included from cmdline.c:23:
  In file included from ./cmdline.h:28:
  ./honggfuzz.h:357:40: error: blocks support disabled - compile with -fblocks or pick a deployment target that supports them
  static inline void defer_cleanup(void (^*b)()) {
                                       ^
  cmdline.c:68:5: error: blocks support disabled - compile with -fblocks or pick a deployment target that supports them
    defer {
    ^
  ./honggfuzz.h:364:39: note: expanded from macro 'defer'
        __attribute__((unused)) void (^defer_varname(__COUNTER__))() = ^
                                      ^
  cmdline.c:68:5: error: blocks support disabled - compile with -fblocks or pick a deployment target that supports them
  ./honggfuzz.h:364:72: note: expanded from macro 'defer'
        __attribute__((unused)) void (^defer_varname(__COUNTER__))() = ^
                                                                       ^
  3 errors generated.
  Makefile:266: recipe for target 'cmdline.o' failed
  make: *** [cmdline.o] Error 1

PS. Hm.. maybe that's the clang version you're using (900)?