Fix build errors and warnings for OpenBSD

c/dirpath.c

@@ -1,3 +1,4 @@
+#include <stdint.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
@@ -101,6 +102,8 @@ char *get_exename(char *buf, size_t size)
     char *p, *path, *pathcpy, filename[PATH_MAX];
     struct stat sbuf;
 
+    (void)size;
+
     pid = getpid();
 
     mib[0] = CTL_KERN;

c/equalhash.c

@@ -1,3 +1,4 @@
+#include <stdarg.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>

c/socket.c

@@ -1,3 +1,5 @@
+#include <sys/select.h>
+
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>

c/utf8.c

@@ -13,8 +13,6 @@
   A UTF-8 validation routine is included.
 */
 
-#define _XOPEN_SOURCE 700
-
 #include <assert.h>
 #include <stdarg.h>
 #include <stddef.h>

scripts/build.sh

@@ -2,7 +2,7 @@
 set -eu
 CC="${CC:-clang}"
 CFLAGS="-Wall -Wextra -Wno-strict-aliasing -std=gnu99"
-CFLAGS="$CFLAGS -O2 -falign-functions"
+CFLAGS="$CFLAGS -O2"  # -falign-functions
 CFLAGS="$CFLAGS -I ../c -D NDEBUG -D USE_COMPUTED_GOTO"
 LFLAGS="-lm"
 builddir="build-$(uname | tr A-Z- a-z_)-$(uname -m | tr A-Z- a-z_)"