SITL logger compile error with GCC 7.3

[MaEtUgR]

Bug
I updated Cygwin to the newest version and it's using GCC 7.3

$ gcc --version
gcc (GCC) 7.3.0

When I compile SITL by make posix I get the following errors which weren't there before on this branch:

../../src/modules/logger/logger.cpp:2136:27: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
statfs_buf.f_bavail >= (px4_statfs_buf_f_bavail_t)(min_free_bytes / statfs_buf.f_bsize)) {
~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

../../src/modules/logger/logger.cpp:2172:26: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
if (statfs_buf.f_bavail < (px4_statfs_buf_f_bavail_t)(50 * 1024 * 1024 / statfs_buf.f_bsize)) {
~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Workaround
Of course I can just add a (int) cast in front of the second operand of the comparison and then it compiles again but I'm writing this issue because there's likely some inconsistency with the types.

Why is px4_statfs_buf_f_bavail_t defined once signed, once usigned here? https://github.com/PX4/Firmware/blob/9ce83f22087311df06078edf3a9da640d93ab396/src/platforms/px4_defines.h#L115-L173

[bkueng]

NuttX uses a different sign from Linux, and apparently Cygwin.

We should something like this (the type is only used in logger):

#ifdef __cplusplus
#include <sys/statfs.h>
typedef decltype(statfs::f_bavail) px4_statfs_buf_f_bavail_t;
#endif

[MaEtUgR]

@bkueng Thanks for your suggestion! What puzzled me most is that this only happens after the last Cygwin update. I'll try/investigate such a solution and create a pr as soon as I found something reasonable.