https://codeberg.org/soundtouch/soundtouch/pulls/34

From 429a4669cfee54ec69f8be61cc4b5ac2d87d1254 Mon Sep 17 00:00:00 2001
From: Sam James <sam@gentoo.org>
Date: Sun, 24 Mar 2024 07:19:34 +0000
Subject: [PATCH 2/2] configure.ac: fix bashism in CXXFLAGS assignment

configure scripts need to be runnable with a POSIX-compliant /bin/sh.

On many (but not all!) systems, /bin/sh is provided by Bash, so errors
like this aren't spotted. Notably Debian defaults to /bin/sh provided
by dash which doesn't tolerate such bashisms as '=='.

This retains compatibility with bash.

Fixes configure warnings/errors like:
```
checking whether make supports nested variables... (cached) yes
configure: 3698: CXXFLAGS+= -Ofast: not found
```

Signed-off-by: Sam James <sam@gentoo.org>
--- a/configure.ac
+++ b/configure.ac
@@ -33,7 +33,7 @@ AC_LANG(C++)
 
 # Compiler flags. Apply -Ofast (implies -O3 -ffast-math) to allow gcc autovectorization
 # generate effective SIMD code.
-CXXFLAGS+=" -Ofast"
+CXXFLAGS="${CXXFLAGS} -Ofast"
 
 # Set AR_FLAGS to avoid build warning "ar: `u' modifier ignored since `D' is the default (see `U')"
 AR_FLAGS='cr'