[lib/LinuxVersion] multiple fixes

• remove fancy but broken static LinuxVersion object.
  This means each usage of a EMPER_LINUX_* macro results in a syscall.
  But I think this is worth it when the result is a sound program.
  Users of those macros should store the result of the comparison anyway.
• fix that we skipped parsing the last part of a version string
• add a test for LinuxVersion
• IO_MUST_INVALIDATE_BROKEN_CHAIN can still be const

emper/lib/LinuxVersion.cpp

@@ -33,19 +33,22 @@ static auto checked_strtol(const std::string& s) -> long {
 
 namespace emper::lib {
 
-LinuxVersion LinuxVersion::linuxVersion;
-
 // Returns 1 if s is smaller, -1 if this is smaller, 0 if equal
 auto LinuxVersion::compare(const std::string& s) -> int {
+	size_t dot_pos, dot_pos2;
 	size_t last_dot_pos = 0, last_dot_pos2 = 0;
 
+	bool was_last = false;
 	for (;;) {
-		size_t dot_pos = this->version.find('.', last_dot_pos);
-		// We run out of parts to compare the versions must be equal
-		if (dot_pos == std::string::npos) return 0;
-
-		size_t dot_pos2 = s.find('.', last_dot_pos2);
-		assert(dot_pos2 != std::string::npos);
+		dot_pos = this->version.find('.', last_dot_pos);
+		if (dot_pos == std::string::npos) {
+			was_last = true;
+			dot_pos = this->version.size();
+			dot_pos2 = s.size();
+		} else {
+			dot_pos2 = s.find('.', last_dot_pos2);
+			assert(dot_pos2 != std::string::npos);
+		}
 
 		long n1 = checked_strtol(this->version.substr(last_dot_pos, dot_pos - last_dot_pos));
 		long n2 = checked_strtol(s.substr(last_dot_pos2, dot_pos2 - last_dot_pos2));
@@ -54,6 +57,9 @@ auto LinuxVersion::compare(const std::string& s) -> int {
 
 		if (n1 < n2) return -1;
 
+		// We ran out of parts to compare the versions must be equal
+		if (was_last) return 0;
+
 		last_dot_pos = dot_pos + 1;
 		last_dot_pos2 = dot_pos2 + 1;
 	}