build: don't override use_llvm/lld defaults

Only set the use_llvm/use_lld options if -Dllvm is actually passed to
zig build.

Fixes: 7cb05dd791b983094487d7cbbea80d5b653512c9

 build.zig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/build.zig b/build.zig
index 428ccc1..c5bbd8e 100644
--- a/build.zig
+++ b/build.zig
@@ -19,7 +19,7 @@ pub fn build(b: *Build) !void {
 
     const strip = b.option(bool, "strip", "Omit debug information") orelse false;
     const pie = b.option(bool, "pie", "Build a Position Independent Executable") orelse false;
-    const use_llvm = b.option(bool, "llvm", "Force use of Zig's LLVM backend and the lld linker") orelse false;
+    const use_llvm = b.option(bool, "llvm", "Force use of Zig's LLVM backend and the lld linker");
 
     const omit_frame_pointer = switch (optimize) {
         .Debug, .ReleaseSafe => false,