Use POSIX-compatible test operators

* `==` is not compatible with stricter POSIX shells
  such as the dash shell.

Bug: https://bugs.gentoo.org/773955

configure.ac

@@ -133,7 +133,7 @@ AC_ARG_ENABLE([legacy],
 AS_IF([test "x$enable_legacy" = "xyes"],
     [AC_DEFINE([ENABLE_LEGACY])])
 
-AM_CONDITIONAL([ENABLE_LEGACY], [test "x$enable_legacy" == "xyes"])
+AM_CONDITIONAL([ENABLE_LEGACY], [test "x$enable_legacy" = "xyes"])
 
 AC_ARG_ENABLE([dsd],
     AS_HELP_STRING([--disable-dsd], [disable support for WavPack DSD files]))
@@ -146,12 +146,12 @@ AM_CONDITIONAL([ENABLE_DSD], [test "x$enable_dsd" != "xno"])
 AC_ARG_ENABLE([rpath],
     AS_HELP_STRING([--enable-rpath], [hardcode library path in executables]))
 
-AM_CONDITIONAL([ENABLE_RPATH], [test "x$enable_rpath" == "xyes"])
+AM_CONDITIONAL([ENABLE_RPATH], [test "x$enable_rpath" = "xyes"])
 
 AC_ARG_ENABLE([tests],
     AS_HELP_STRING([--enable-tests], [build libwavpack test program (requires Pthreads)]))
 
-AM_CONDITIONAL([ENABLE_TESTS], [test "x$enable_tests" == "xyes"])
+AM_CONDITIONAL([ENABLE_TESTS], [test "x$enable_tests" = "xyes"])
 
 AC_ARG_ENABLE([asm],
     [AS_HELP_STRING([--disable-asm], [disable assembly optimizations])],,