Fix build on older glibc versions.

_POSIX_C_SOURCE is necessary for getopt(3), and _DEFAULT_SOURCE is
necessary for the d_type enumerations in struct dirent.

Since we are here, use the C11 standard explicitly.

These fixes only allow us to build with glibc>=2.28, which is when the
<threads.h> header was added [1], necessary to access the thread_local
macro.

[1] https://stackoverflow.com/a/22875599

Makefile

@@ -1,5 +1,5 @@
 OPT = -O2
-CFLAGS = -Wall -Wextra $(OPT)
+CFLAGS = -std=c11 -D_POSIX_C_SOURCE=200809UL -D_DEFAULT_SOURCE -Wall -Wextra $(OPT)
 
 .PHONY: all