Fix compilation under glibc.

Some feature macros behaved differently from the ones in musl.

gemi.c

@@ -1,3 +1,4 @@
+#define _XOPEN_SOURCE /* getopt */
 #include <stdio.h>
 #include <stdlib.h>
 #include <signal.h>

mmap_file.c

@@ -1,3 +1,4 @@
+#define _DEFAULT_SOURCE /* MAP_ANONYMOUS */
 #include <assert.h>
 #include <errno.h>
 #include <stdio.h>
@@ -11,6 +12,9 @@
 
 #include "mmap_file.h"
 
+const int PROT_MEM = PROT_WRITE | PROT_READ;
+const int MAP_MEM = MAP_ANONYMOUS | MAP_PRIVATE;
+
 bool allocate_mmap(struct mmap_file *f)
 {
     f->data = mmap(NULL, f->size, f->prot, f->flags, -1, 0);

mmap_file.h

@@ -10,8 +10,8 @@
 #define ERROR_MMAP(file) ((file).data == MAP_FAILED || (file).data == NULL)
 
 // definitions for memory backed mappings
-#define PROT_MEM (PROT_WRITE | PROT_READ)
-#define MAP_MEM (MAP_ANONYMOUS | MAP_PRIVATE)
+extern const int PROT_MEM;
+extern const int MAP_MEM;
 
 // 128KiB
 #define OUTPUT_FILE_SIZE (128 * 1024)

purr.c

@@ -1,7 +1,9 @@
+#define _XOPEN_SOURCE /* getopt */
+
 #ifdef HAVE_PROG_INVOCATION
-#define _GNU_SOURCE
+#define _GNU_SOURCE /* program_invocation_short_name */
 #include <errno.h>
-#endif
+#endif /* HAVE_PROG_INVOCATION */
 
 #include <signal.h>
 #include <stdbool.h>

urls.c

@@ -1,3 +1,4 @@
+#define _POSIX_C_SOURCE 200112L /* addrinfo */
 #include <string.h>
 #include <unistd.h>
 #include <stdlib.h>