Fix ZTS build for Alpine (musl libc)

Author: mvorisek

configure.ac

@@ -260,7 +260,7 @@ AC_MSG_CHECKING([whether we are using musl libc])
 if command -v ldd >/dev/null && ldd --version 2>&1 | grep -q ^musl
 then
   AC_MSG_RESULT(yes)
-  CPPFLAGS="$CPPFLAGS -D__MUSL__"
+  AC_DEFINE([__MUSL__], [1], [Define when using musl libc])
 else
   AC_MSG_RESULT(no)
 fi