SDL: build: Fix the LINKER variable.

SDL Commits
1 
From d2e8998f808b4ddab1decd2491a9f2e7692c6d53 Mon Sep 17 00:00:00 2001
From: orbea <[EMAIL REDACTED]>
Date: Tue, 3 Aug 2021 07:59:01 -0700
Subject: [PATCH] build: Fix the LINKER variable.

The LINKER variable is set in configure.ac as either 'CC' or 'CXX'
where it is then passed to the created Makefile. This fails with
slibtool which can't find the 'CC' file and can be fixed by correctly
setting the LINKER variable to an actual Makefile variable like '$(CC)'
or '$(CXX)' instead. Presumably GNU libtool does some magic here to
hide the issue.
---
 configure.ac | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/configure.ac b/configure.ac
index 82973c5db..1e5d571a3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -84,10 +84,10 @@ esac
 dnl use CXX for linker on Haiku
 case "$host" in
     *-*-haiku*)
-        LINKER=CXX
+        LINKER='$(CXX)'
         ;;
     *)
-        LINKER=CC
+        LINKER='$(CC)'
         ;;
 esac
 AC_SUBST(LINKER)