Remove LFS64 calls and set _FILE_OFFSET_BITS=64

Musl 1.2.4 made the LFS64 interfaces only available when
_LARGEFILE64_SOURCE is defined, and they will be removed altogether in
Musl 1.2.5. This commit replaces the LFS64 calls with their non-LFS64
versions and defines _FILE_OFFSET_BITS=64, which makes all interfaces
64-bit.

Bug: https://bugs.gentoo.org/905999

CMakeLists.txt

@@ -31,6 +31,7 @@ target_compile_definitions(
     PUBLIC $<$<CONFIG:Debug>:TINYXML2_DEBUG>
     INTERFACE $<$<BOOL:${BUILD_SHARED_LIBS}>:TINYXML2_IMPORT>
     PRIVATE $<$<CXX_COMPILER_ID:MSVC>:_CRT_SECURE_NO_WARNINGS>
+    PUBLIC _FILE_OFFSET_BITS=64
 )
 
 set_target_properties(