configure.ac: correctly manage llvm auto-detection

Earlier refactoring commits changed from one, dare I say it, broken
behaviour to another. Namely:

Before, as you explicitly --enable-gallium-llvm your selection was
ignored when llvm-config was not present/detected.
Today, the "auto" heuristics enables gallium llvm regardless if you have
llvm/llvm-config available or not.

Rework the auto-detection to attribute for llvm's presence.

v2: Set enable_gallium_llvm=no when LLVM is not found.

Cc: "17.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Tobias Droste <tdroste@gmx.de>
Reported-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Tested-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit d4840c0c26315fe4f0ba82e2b0d81cd9f2636f5e)
diff --git a/configure.ac b/configure.ac
index d4c1fd4..8448736 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1744,10 +1744,14 @@
     [enable_gallium_llvm=auto])
 
 if test "x$enable_gallium_llvm" = xauto; then
-    case "$host_cpu" in
-    i*86|x86_64|amd64) enable_gallium_llvm=yes;;
-    *) enable_gallium_llvm=no;;
-    esac
+    if test "x$FOUND_LLVM" = xyes; then
+        case "$host_cpu" in
+        i*86|x86_64|amd64) enable_gallium_llvm=yes;;
+        *) enable_gallium_llvm=no;;
+        esac
+    else
+        enable_gallium_llvm=no
+    fi
 fi
 
 if test "x$enable_gallium_llvm" = xyes -a "x$FOUND_LLVM" = xno; then