Fix bug in configure.ac which breaks in Alpine

Brace usage is incorrect which leads to compile failure in Alpine.

Author: LinuxJedi

configure.ac

@@ -12,7 +12,7 @@ AC_CONFIG_AUX_DIR([build-aux])
 : ${CFLAGS=""}
 
 # Test ar for the "U" option. Should be checked before the libtool macros.
-xxx_ar_flags=$((ar --help) 2>&1)
+xxx_ar_flags=$(ar --help 2>&1)
 AS_CASE([$xxx_ar_flags],[*'use actual timestamps and uids/gids'*],[: ${AR_FLAGS="Ucru"}])
 
 # Checks for programs.