Support older linux systems (#545)

* Fix pal_linux.h for older linux systems

Where MADV_FREE is not defined - replaced with MADV_DONTNEED
Where GRND_NONBLOCK is not defined in <sys/random.h> but in <linux/random.h>

* Check for linux/random.h in CMake

as __has_include seems to not be reliable

* Use CMake module CheckIncludeFilesCXX

as C language isn't enabled by default everywhere

* Move madvise flag ifdefs into constexpr for cleaner code

src/snmalloc/pal/pal_linux.h

@@ -8,6 +8,11 @@
 #  include <sys/mman.h>
 #  include <sys/prctl.h>
 #  include <syscall.h>
+// __has_include does not reliably determine if we actually have linux/random.h
+// available
+#  if defined(SNMALLOC_HAS_LINUX_RANDOM_H)
+#    include <linux/random.h>
+#  endif
 
 extern "C" int puts(const char* str);
 
@@ -35,6 +40,19 @@ namespace snmalloc
      */
     static constexpr int default_mmap_flags = MAP_NORESERVE;
 
+    /**
+     * MADV_FREE is only available since Linux 4.5.
+     *
+     * Fallback to MADV_DONTNEED on older kernels
+     */
+    static constexpr int madvise_free_flags =
+#  ifdef SNMALLOC_HAS_LINUX_RANDOM_H
+      MADV_FREE
+#  else
+      MADV_DONTNEED
+#  endif
+      ;
+
     static void* reserve(size_t size) noexcept
     {
       void* p = PALPOSIX<PALLinux>::reserve(size);
@@ -108,7 +126,7 @@ namespace snmalloc
         memset(p, 0x5a, size);
 
       madvise(p, size, MADV_DONTDUMP);
-      madvise(p, size, MADV_FREE);
+      madvise(p, size, madvise_free_flags);
 
       if constexpr (PalEnforceAccess)
       {