Fix PAGESIZE to compile on systems where it's already defined as a macro.

[creationix]

On some systems PAGESIZE is a macro that expands to a numeric constant. This causes a syntax error.

/tmp/lua-resty-simdjson-1.2.0 # make libsimdjson_ffi.o
c++ -ggdb -O3 -DNDEBUG -o libsimdjson_ffi.o  -c -fPIC src/simdjson_ffi.cpp
In file included from /usr/include/limits.h:40,
                 from /usr/include/c++/13.2.1/climits:42,
                 from src/simdjson.h:136,
                 from src/simdjson_ffi.cpp:1:
src/simdjson_ffi.cpp:9:13: error: expected unqualified-id before numeric constant
    9 | static long PAGESIZE = 0;
      |             ^~~~~~~~
src/simdjson_ffi.cpp: In function 'bool need_allocation(const char*, std::size_t)':
src/simdjson_ffi.cpp:20:9: error: lvalue required as left operand of assignment
   20 |         PAGESIZE = getpagesize();
      |         ^~~~~~~~
make: *** [Makefile:40: libsimdjson_ffi.o] Error 1

This PR fixes this by using ifndef to only try to initialize and set it if the macro doesn't exist.

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