Set _X_OPEN_SOURCE and default compiler in build script

c/string.c

@@ -2,8 +2,6 @@
   string functions
 */
 
-#define _XOPEN_SOURCE
-
 #include <sys/types.h>
 #include <sys/time.h>
 

c/time_unix.c

@@ -1,5 +1,3 @@
-#define _XOPEN_SOURCE
-
 #include <sys/poll.h>
 #include <sys/time.h>
 

c/utf8.c

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

scripts/build.sh

@@ -1,11 +1,42 @@
 #!/bin/sh
 set -eu
-CC="${CC:-clang}"
 CFLAGS="-Wall -Wextra -Wno-strict-aliasing -std=gnu99"
 CFLAGS="$CFLAGS -O2" # -falign-functions
 CFLAGS="$CFLAGS -I ../c -D NDEBUG -D USE_COMPUTED_GOTO"
 LFLAGS="-lm"
 os="$(uname | tr A-Z- a-z_)"
+case "$os" in
+darwin)
+    default_cc="clang"
+    ;;
+dragonfly)
+    default_cc="gcc"
+    ;;
+freebsd)
+    default_cc="clang"
+    ;;
+haiku)
+    default_cc="gcc"
+    ;;
+linux)
+    default_cc="gcc"
+    CFLAGS="$CFLAGS -D _XOPEN_SOURCE"
+    ;;
+netbsd)
+    default_cc="gcc"
+    ;;
+openbsd)
+    default_cc="clang"
+    ;;
+sunos)
+    default_cc="gcc"
+    ;;
+*)
+    echo "Unknown operating system: $os" >&2
+    exit 1
+    ;;
+esac
+CC="${CC:-$default_cc}"
 builddir="build-$os-$(uname -m | tr A-Z- a-z_)"
 cd "$(dirname "$0")"/..
 echo "Entering directory '$PWD'"