diff --git a/c/string.c b/c/string.c
index b29ea5f..9d83e3e 100644
--- a/c/string.c
+++ b/c/string.c
@@ -2,8 +2,6 @@
   string functions
 */
 
-#define _XOPEN_SOURCE
-
 #include <sys/types.h>
 #include <sys/time.h>
 
diff --git a/c/time_unix.c b/c/time_unix.c
index 4cca98b..d48da3f 100644
--- a/c/time_unix.c
+++ b/c/time_unix.c
@@ -1,5 +1,3 @@
-#define _XOPEN_SOURCE
-
 #include <sys/poll.h>
 #include <sys/time.h>
 
diff --git a/c/utf8.c b/c/utf8.c
index fb94776..8e4ff59 100644
--- a/c/utf8.c
+++ b/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>
diff --git a/scripts/build.sh b/scripts/build.sh
index b12f00b..702586d 100755
--- a/scripts/build.sh
+++ b/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'"
@@ -41,11 +72,11 @@ $CC $CFLAGS -c ../c/table.c
 $CC $CFLAGS -c ../c/time_unix.c
 $CC $CFLAGS -c ../c/utf8.c
 $CC $LFLAGS -o flisp -lm \
-        bitvector-ops.o bitvector.o builtins.o dump.o env_unix.o \
-        equalhash.o flisp.o flmain.o fs_"$os".o fs_unix.o \
-        hashing.o htable.o int2str.o \
-        ios.o iostream.o lltinit.o ptrhash.o random.o socket.o \
-        string.o table.o time_unix.o utf8.o
+    bitvector-ops.o bitvector.o builtins.o dump.o env_unix.o \
+    equalhash.o flisp.o flmain.o fs_"$os".o fs_unix.o \
+    hashing.o htable.o int2str.o \
+    ios.o iostream.o lltinit.o ptrhash.o random.o socket.o \
+    string.o table.o time_unix.o utf8.o
 { set +x; } 2>/dev/null
 cd ../scheme-core
 echo "Entering directory '$PWD'"