[LinuxVersion] Use "Construct on first use" idiom and add mutex

Use the "Construct on first use" idom for UTS Release (aka Linux version) initialization. The previous implementation was fragle and could lead to segfaults like

__s2=0x7fffffffe480 "5.16.10-arch1-1", __s1=<optimized out>)
at /usr/include/c++/11.2.0/bits/char_traits.h:409
__n=<optimized out>, __s=0x7fffffffe480 "5.16.10-arch1-1", __d=<optimized out>)
at /usr/include/c++/11.2.0/bits/basic_string.h:359
__n=<optimized out>, __s=0x7fffffffe480 "5.16.10-arch1-1", __d=<optimized out>)
at /usr/include/c++/11.2.0/bits/basic_string.h:354
__str=..., this=0x7ffff7fbd2a0 <emper::lib::LinuxVersion::globalVersion[abi:cxx11]>)
at /usr/include/c++/11.2.0/bits/basic_string.h:739
at ../emper/lib/LinuxVersion.cpp:46
at /usr/include/c++/11.2.0/ext/new_allocator.h:79

if globalVersion's constructor was not yet called. This is the so called "static initialization order fiasco" in C++.

While the mutex may not be strictly requires, as we do not call LinuxVersion's non-argument constructor nor getUtsRelease() concurrently, it can not hurt to have one.

Closes !347 (closed).

emper/lib/LinuxVersion.cpp

 // SPDX-License-Identifier: LGPL-3.0-or-later
-// Copyright © 2021 Florian Fischer
+// Copyright © 2021-2022 Florian Fischer, Florian Schmaus
 #include "lib/LinuxVersion.hpp"
 
 #include <sys/utsname.h>
 
 #include <cerrno>
 #include <cstdlib>
+#include <mutex>
 
 #include "Common.hpp"
 
@@ -35,19 +36,30 @@ static auto checked_strtol(const std::string& s) -> long {
 
 namespace emper::lib {
 
-std::string LinuxVersion::globalVersion;
-
-LinuxVersion::LinuxVersion() {
-	if (globalVersion.empty()) {
-		struct utsname buf;
-		if (uname(&buf)) {
-			DIE_MSG_ERRNO("Failed to invoke uname()");
+auto LinuxVersion::getUtsRelease() -> const std::string& {
+	// Use "construct (members) on first use idiom" to prevent the "static initialization order
+	// fiasco/problem".
+	static std::string utsRelease;
+	// This mutex will become necessary once we call getUtsRelease(),
+	// and by implication LinuxVersion's constructor, concurrently. So
+	// it can not hurt to have it right now.
+	static std::mutex utsReleaseMutex;
+
+	{
+		const std::lock_guard<std::mutex> lock(utsReleaseMutex);
+		if (utsRelease.empty()) {
+			struct utsname buf;
+			if (uname(&buf)) {
+				DIE_MSG_ERRNO("Failed to invoke uname()");
+			}
+			utsRelease = std::string(buf.release);
 		}
-		globalVersion = std::string(buf.release);
 	}
-	version = globalVersion;
+	return utsRelease;
 }
 
+LinuxVersion::LinuxVersion() { version = getUtsRelease(); }
+
 // 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;