[libaxx] add cmath and use cmath instead of math.h when required

Change-Id: Id839b17d33c69e2e002f370e553ff35246a1bc90

poincare/src/variable_context.cpp

@@ -1,6 +1,9 @@
 #include <poincare/variable_context.h>
 #include <poincare/preferences.h>
 #include <assert.h>
+extern "C" {
+#include <math.h>
+}
 
 namespace Poincare {