Provide option to CMake for Page Size (#664)

* Pickup page size from unistd.h

This uses the PAGESIZE constant from the unistd.h on POSIX.
This should make the code more resilient to being compiled on platforms with
different page sizes.

* Allow pagesize to come from cmake.

* Update src/snmalloc/pal/pal_posix.h

Co-authored-by: Nathaniel Filardo <105816689+nwf-msr@users.noreply.github.com>

---------

Co-authored-by: Nathaniel Filardo <105816689+nwf-msr@users.noreply.github.com>

src/snmalloc/pal/pal_posix.h

@@ -8,6 +8,7 @@
 #endif
 #include <errno.h>
 #include <fcntl.h>
+#include <limits.h>
 #include <stdio.h>
 #include <string.h>
 #include <strings.h>
@@ -129,8 +130,15 @@ namespace snmalloc
       | Entropy
 #endif
       ;
-
+#ifdef SNMALLOC_PAGESIZE
+    static_assert(
+      bits::is_pow2(SNMALLOC_PAGESIZE), "Page size must be a power of 2");
+    static constexpr size_t page_size = SNMALLOC_PAGESIZE;
+#elif defined(PAGESIZE)
+    static constexpr size_t page_size = max(Aal::smallest_page_size, PAGESIZE);
+#else
     static constexpr size_t page_size = Aal::smallest_page_size;
+#endif
 
     /**
      * Address bits are potentially mediated by some POSIX OSes, but generally