Make LLVM compile on DragonFly BSD (PR2499).

Patch by Hasso Tepper!

llvm-svn: 52781
diff --git a/llvm/lib/Target/CBackend/CBackend.cpp b/llvm/lib/Target/CBackend/CBackend.cpp
index 2e9902d..71895f4 100644
--- a/llvm/lib/Target/CBackend/CBackend.cpp
+++ b/llvm/lib/Target/CBackend/CBackend.cpp
@@ -1415,7 +1415,7 @@
       << "extern void *__builtin_alloca(unsigned int);\n"
       << "#endif\n"
       << "#define alloca(x) __builtin_alloca(x)\n"
-      << "#elif defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__)\n"
+      << "#elif defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__) || defined(__DragonFly__)\n"
       << "#define alloca(x) __builtin_alloca(x)\n"
       << "#elif defined(_MSC_VER)\n"
       << "#define inline _inline\n"