diff --git a/libpcc/bitcnt.c b/libpcc/bitcnt.c
index 26ecc01..c3bd54e 100644
--- a/libpcc/bitcnt.c
+++ b/libpcc/bitcnt.c
@@ -1,9 +1,22 @@
+// Only use __has_builtin in compilers that support it.
+#ifndef __has_builtin
+	#define __has_builtin(x) 0
+#endif
 
+#if !__has_builtin(__builtin_clz)
 int __builtin_clz(unsigned int);
+#endif
+#if !__has_builtin(__builtin_ctz)
 int __builtin_ctz(unsigned int);
+#endif
+#if !__has_builtin(__builtin_clzl)
 int __builtin_clzl(unsigned long);
+#endif
+#if !__has_builtin(__builtin_ctzl)
 int __builtin_ctzl(unsigned long);
+#endif
 
+#if !__has_builtin(__builtin_clz)
 int
 __builtin_clz(unsigned int v)
 {
@@ -14,7 +27,9 @@ __builtin_clz(unsigned int v)
 			break;
 	return i;
 }
+#endif
 
+#if !__has_builtin(__builtin_ctz)
 int
 __builtin_ctz(unsigned int v)
 {
@@ -25,7 +40,9 @@ __builtin_ctz(unsigned int v)
 			break;
 	return i;
 }
+#endif
 
+#if !__has_builtin(__builtin_clzl)
 int
 __builtin_clzl(unsigned long v)
 {
@@ -37,7 +54,9 @@ __builtin_clzl(unsigned long v)
 			break;
 	return i;
 }
+#endif
 
+#if !__has_builtin(__builtin_ctzl)
 int
 __builtin_ctzl(unsigned long v)
 {
@@ -48,3 +67,4 @@ __builtin_ctzl(unsigned long v)
 			break;
 	return i;
 }
+#endif