gallivm: limit use of setFastMathFlags to LLVM 3.8 and later

Reviewed-by: Brian Paul <brianp@vmware.com>

src/gallium/auxiliary/gallivm/lp_bld_misc.cpp

@@ -739,11 +739,13 @@ lp_create_builder(LLVMContextRef ctx, bool unsafe_fpmath)
 {
    LLVMBuilderRef builder = LLVMCreateBuilderInContext(ctx);
 
+#if HAVE_LLVM >= 0x0308
    if (unsafe_fpmath) {
       llvm::FastMathFlags flags;
       flags.setUnsafeAlgebra();
       llvm::unwrap(builder)->setFastMathFlags(flags);
    }
+#endif
 
    return builder;
 }