[gcc r12-2686] Fix bashism in `libsanitizer/configure.tgt'

[Martin Liska <marxin@gcc.gnu.org>]

https://gcc.gnu.org/g:759f3854f0fdb4add2961bfafd1ee793f392f70a

commit r12-2686-g759f3854f0fdb4add2961bfafd1ee793f392f70a
Author: Mosè Giordano <mose@gnu.org>
Date:   Fri Jun 18 23:46:44 2021 +0000

    Fix bashism in `libsanitizer/configure.tgt'
    
    Appending to a string variable with `+=' is a bashism and does not work in
    strict POSIX shells like dash.  This results in the extra compilation flags not
    to be set correctly.  This patch replaces the `+=' syntax with a simple string
    interpolation to append to the `EXTRA_CXXFLAGS' variable.
    
    libsanitizer/ChangeLog
    
            PR sanitizer/101111
            * configure.tgt: Fix bashism in setting of `EXTRA_CXXFLAGS'.

Diff:
---
 libsanitizer/configure.tgt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libsanitizer/configure.tgt b/libsanitizer/configure.tgt
index f635e412bdc..5a59ea6a1b5 100644
--- a/libsanitizer/configure.tgt
+++ b/libsanitizer/configure.tgt
@@ -70,7 +70,7 @@ case "${target}" in
 	;;
   x86_64-*-darwin2* | x86_64-*-darwin1[2-9]* | i?86-*-darwin1[2-9]*)
 	TSAN_SUPPORTED=no
-	EXTRA_CXXFLAGS+="-Wl,-undefined,dynamic_lookup"
+	EXTRA_CXXFLAGS="${EXTRA_CXXFLAGS} -Wl,-undefined,dynamic_lookup"
 	;;
   x86_64-*-solaris2.11* | i?86-*-solaris2.11*)
 	;;