Merge branch 'jc/freebsd-without-c99-only-build'

FreeBSD 13.0 headers have unconditional dependency on C11 language
features, and adding -std=gnu99 to DEVELOPER_CFLAGS would just
break the developer build.

* jc/freebsd-without-c99-only-build:
  Makefile: FreeBSD cannot do C99-or-below build

config.mak.dev

@@ -20,9 +20,14 @@ endif
 endif
 endif
 
+ifneq ($(uname_S),FreeBSD)
 ifneq ($(or $(filter gcc6,$(COMPILER_FEATURES)),$(filter clang7,$(COMPILER_FEATURES))),)
 DEVELOPER_CFLAGS += -std=gnu99
 endif
+else
+# FreeBSD cannot limit to C99 because its system headers unconditionally
+# rely on C11 features.
+endif
 
 DEVELOPER_CFLAGS += -Wdeclaration-after-statement
 DEVELOPER_CFLAGS += -Wformat-security