cmake: check fseeko after detecting HAVE_FILE_OFFSET_BITS

[SirLynix]

On Android, fseeko is defined as is (I omitted other functions for clarity):

/* See https://android.googlesource.com/platform/bionic/+/master/docs/32-bit-abi.md */
#if defined(__USE_FILE_OFFSET64)

#if __ANDROID_API__ >= 24
int fseeko(FILE* _Nonnull __fp, off_t __offset, int __whence) __RENAME(fseeko64) __INTRODUCED_IN(24);
#endif /* __ANDROID_API__ >= 24 */

#else
int fseeko(FILE* _Nonnull __fp, off_t __offset, int __whence);

#if __ANDROID_API__ >= 24
int fseeko64(FILE* _Nonnull __fp, off64_t __offset, int __whence) __INTRODUCED_IN(24);
#endif /* __ANDROID_API__ >= 24 */

#endif

Which means that on Android 32bits (armv7), fseeko is only available in 32bits mode, if __USE_FILE_OFFSET64 (_FILE_OFFSET_BITS=64) is set then fseeko is not available.

CMake detects fseeko as available because _FILE_OFFSET_BITS=64 is not yet defined when it tests for this function existence, but then it defines _FILE_OFFSET_BITS=64 which makes fseeko unavailable and fails compilation:

/home/runner/.xmake/cache/packages/2404/l/libcurl/8.6.0/source/lib/formdata.c:794:10: error: call to undeclared function 'fseeko'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
  return fseeko(stream, (off_t)offset, whence);
         ^
/home/runner/.xmake/cache/packages/2404/l/libcurl/8.6.0/source/lib/formdata.c:794:10: note: did you mean 'fseek'?
/home/runner/work/xmake-repo/xmake-repo/android-ndk-r26b/toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/include/stdio.h:215:5: note: 'fseek' declared here
int fseek(FILE* _Nonnull __fp, long __offset, int __whence);
    ^

A simple fix is to test for fseeko existence after testing HAVE_FILE_OFFSET_BITS and especially after

  set(CMAKE_REQUIRED_FLAGS "-D_FILE_OFFSET_BITS=64")

tested here

Fix #12086 for good

[bagder]

Thanks!