Tolerate open and fopen being called before our own constructor.

An easy example of this is coreutils' "ls", when linked against libselinux:
libselinux will/may call fopen("/proc/filesystems", "re") during its own
constructor, which may be called before our.
Another example is inkscape, which contains static C++ classes (such as
Inkscape::Util::UnitTable::UnitTable) which may themselves call (f)open.
C++ runtime seems to call such static constructors before our contructor,
leading to the same issue. GCC documentation (as of version 8.0.1) says:
  However, at present, the order in which constructors for C++ objects
  with static storage duration and functions decorated with attribute
  constructor are invoked is unspecified. In mixed declarations,
  attribute init_priority can be used to impose a specific ordering.

README.rst

@@ -20,7 +20,7 @@ Requirements
 ------------
 
 - make
-- gcc (source uses __attribute__)
+- a C compiler
 - glibc (source uses RTLD_NEXT)
 
 Building

userhosts.c

@@ -56,7 +56,7 @@ static inline void *dlsym_or_abort(const char *name) {
   return symbol;
 }
 
-static void __attribute__ ((constructor)) init(void) {
+static void init(void) {
   original_open = dlsym_or_abort("open");
   original_fopen = dlsym_or_abort("fopen");
   replacement_hosts = getenv("HOSTS");
@@ -67,6 +67,8 @@ static void __attribute__ ((constructor)) init(void) {
 }
 
 int open(const char *__file, int __oflag, ...) {
+  if (!original_open)
+    init();
   if (!strcmp(__file, ORIGINAL_HOSTS_PATH))
     __file = replacement_hosts;
   if (__oflag & (O_CREAT | PURE_O_TMPFILE)) {
@@ -82,6 +84,8 @@ int open(const char *__file, int __oflag, ...) {
 }
 
 FILE *fopen(const char *path, const char *mode) {
+  if (!original_fopen)
+    init();
   if (!strcmp(path, ORIGINAL_HOSTS_PATH))
     path = replacement_hosts;
   return (*original_fopen)(path, mode);