configure.ac: honour LLVM_LIBDIR when linking against LLVM

Currently if one uses a non-default prefix, the path won't get
propagated and we'll fail at link-time.

A very quick and easy example is to install to /usr/local.
At this point, llvm-config will be picked even without the
--with-llvm-prefix, but regardless of the latter linking will fail.

Currently people can workaround that via LD_LIBRARY_PATH.

Cc: "12.0 13.0" <mesa-stable@lists.freedesktop.org>
Cc: Tom Stellard <thomas.stellard@amd.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

configure.ac

@@ -1985,8 +1985,6 @@ AC_SUBST([LLVM_BINDIR])
 AC_SUBST([LLVM_CFLAGS])
 AC_SUBST([LLVM_CPPFLAGS])
 AC_SUBST([LLVM_CXXFLAGS])
-AC_SUBST([LLVM_LIBDIR])
-AC_SUBST([LLVM_LIBS])
 AC_SUBST([LLVM_LDFLAGS])
 AC_SUBST([LLVM_INCLUDEDIR])
 AC_SUBST([LLVM_VERSION])
@@ -2542,7 +2540,9 @@ if test "x$MESA_LLVM" != x0; then
             LLVM_LIBS="$LLVM_LIBS `$LLVM_CONFIG --system-libs`"
         fi
     fi
+    LLVM_LIBS="$LLVM_LIBDIR $LLVM_LIBS"
 fi
+AC_SUBST([LLVM_LIBS])
 
 AM_CONDITIONAL(HAVE_GALLIUM_SVGA, test "x$HAVE_GALLIUM_SVGA" = xyes)
 AM_CONDITIONAL(HAVE_GALLIUM_I915, test "x$HAVE_GALLIUM_I915" = xyes)