Fix DJGPP build

GCC with the -std=c11 defines __STRICT_ANSI__. DJGPP checks if
__STRICT_ANSI__ is defined and if so doesn't define some things
mandated by POSIX such as struct stat, PATH_MAX, and others.
The -std=gnu11 option does not define this macro, so use it instead.

_DEFAULT_SOURCE isn't needed as no GNU nor BSD-specific functions
are used. Remove it.

Fix the last two occurrences of incorrect format specifiers for standard
fixed-width integer types.

Makefile

@@ -34,8 +34,8 @@ WARNFLAGS	:= -Wall
 # Overridable CFLAGS
 CFLAGS		:= -O3
 # Non-overridable CFLAGS
-REALCFLAGS	:= ${CFLAGS} ${WARNFLAGS} -std=c11 -D_POSIX_C_SOURCE=200809L \
-		   -D_DEFAULT_SOURCE -Iinclude
+REALCFLAGS	:= ${CFLAGS} ${WARNFLAGS} -std=gnu11 -D_POSIX_C_SOURCE=200809L \
+		   -Iinclude
 # Overridable LDFLAGS
 LDFLAGS		:=
 # Non-overridable LDFLAGS