Add static_assert verifing page size is constexpr.

Whenever PAGE_SIZE is defined, make sure that the getPageSizeCached()
function is constexpr using a static_assert. If it isn't this won't
compile.

Test: treehugger
Change-Id: I4f7374cc6d9d7b538d7bf33a0748ba17b13b6218

config/custom_scudo_config.h

@@ -220,6 +220,13 @@ struct AndroidLowMemoryConfig {
 
 #if defined(__ANDROID__)
 
+#include <unistd.h>
+
+#if defined(PAGE_SIZE)
+// This is to guarantee that the getPageSizeCached() function is constexpr.
+static_assert(getPageSizeCached() != 0, "getPageSizeCached() is zero");
+#endif
+
 #if defined(SCUDO_LOW_MEMORY)
 typedef AndroidLowMemoryConfig Config;
 #else