Require that per-feature maxzoom and minzoom be small integers

Also fixes a place in the Makefile that accidentally still used
CFLAGS instead of CXXFLAGS

text.cpp

@@ -1,5 +1,7 @@
 #include "text.hpp"
 #include <stdio.h>
+#include <math.h>
+#include <stdlib.h>
 
 /**
  * Returns an empty string if `s` is valid utf8;
@@ -122,3 +124,12 @@ std::string truncate16(std::string const &s, size_t runes) {
 
 	return std::string(s, 0, lastgood - start);
 }
+
+int integer_zoom(std::string where, std::string text) {
+	double d = atof(text.c_str());
+	if (!isnormal(d) || d != floor(d) || d < 0 || d > 32) {
+		fprintf(stderr, "%s: Expected integer zoom level in \"tippecanoe\" GeoJSON extension, not %s\n", where.c_str(), text.c_str());
+		exit(EXIT_FAILURE);
+	}
+	return d;
+}