SDL: render: Fix detection of fxc, dxc in build-shaders.sh

SDL Commits
1 
From 13500bbf508f2ce98a1abccfb1539616be6694d9 Mon Sep 17 00:00:00 2001
From: Simon McVittie <[EMAIL REDACTED]>
Date: Tue, 17 Sep 2024 11:43:08 +0100
Subject: [PATCH] render: Fix detection of fxc, dxc in build-shaders.sh

We want `$USE_FXC` to default to the result of evaluating the variable
`$HAVE_FXC`, not the literal string `HAVE_FXC`, and the same for dxc.

Signed-off-by: Simon McVittie <smcv@debian.org>
---
 src/render/gpu/shaders/build-shaders.sh | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/render/gpu/shaders/build-shaders.sh b/src/render/gpu/shaders/build-shaders.sh
index 3345a0bc1f927..81e45cb479f61 100755
--- a/src/render/gpu/shaders/build-shaders.sh
+++ b/src/render/gpu/shaders/build-shaders.sh
@@ -10,8 +10,8 @@ which dxc &>/dev/null && HAVE_DXC=1 || HAVE_DXC=0
 [ "$HAVE_FXC" != 0 ] || echo "fxc not in PATH; D3D11 shaders will not be rebuilt"
 [ "$HAVE_DXC" != 0 ] || echo "dxc not in PATH; D3D12 shaders will not be rebuilt"
 
-USE_FXC=${USE_FXC:-HAVE_FXC}
-USE_DXC=${USE_DXC:-HAVE_DXC}
+USE_FXC=${USE_FXC:-$HAVE_FXC}
+USE_DXC=${USE_DXC:-$HAVE_DXC}
 
 spirv_bundle="spir-v.h"
 dxbc50_bundle="dxbc50.h"