improve log2 test

- add check for native log2 to configure
- use a common define (NOT_HAVE_LOG2) to enable use of local library
  version for non-autoconf platforms without their own version,
  currently msvc and android

This uses a negative (NOT_HAVE_) to simplify the ifdef

Change-Id: Id0610eed507f8bb9c5da338918112853d5c8127a

configure.ac

@@ -11,6 +11,11 @@ AC_ARG_WITH([pkgconfigdir], AS_HELP_STRING([--with-pkgconfigdir=PATH],
 	[pkgconfigdir="$withval"], [pkgconfigdir='${libdir}/pkgconfig'])
 AC_SUBST([pkgconfigdir])
 
+dnl === Check for native log2
+AC_SEARCH_LIBS([log2], [m],,
+               [AC_DEFINE([NOT_HAVE_LOG2], [1],
+                          [Undefine this if you have log2().])])
+
 dnl === Check libz is present
 
 if test "$enable_experimental" = "yes"; then