[giflib]fix compile error in x64-uwp

[JackBoosY]

Installing giflib will prompt some functions to be unsafe, so I added macro -D_CRT_SECURE_NO_WARNINGS and replaced some safe functions to avoid compilation failure.

See #3043.

[JackBoosY]

@Rastaban I have modified code, please check it out. Thank you.

[Rastaban]

Resolved merge conflict

[qis]

@JackBoosY @Rastaban Could you please revert this change? It breaks compilation on Linux. Tested on Ubuntu 18.04.2 LTS.

Edit: I mean "you can't link against it". It compiles just fine, which is why it probably slipped by.

/opt/vcpkg/installed/x64-linux/debug/lib/libgif.a(egif_lib.c.o): In function `EGifOpenFileName':
/opt/vcpkg/buildtrees/giflib/src/5.1.4-8e625bd21e/lib/egif_lib.c:63: undefined reference to `_open'
/opt/vcpkg/buildtrees/giflib/src/5.1.4-8e625bd21e/lib/egif_lib.c:66: undefined reference to `_open'
/opt/vcpkg/buildtrees/giflib/src/5.1.4-8e625bd21e/lib/egif_lib.c:76: undefined reference to `_close'
/opt/vcpkg/installed/x64-linux/debug/lib/libgif.a(egif_lib.c.o): In function `EGifOpenFileHandle':
/opt/vcpkg/buildtrees/giflib/src/5.1.4-8e625bd21e/lib/egif_lib.c:121: undefined reference to `_fdopen'

Here is the "workaround" I have to use now:

https://github.com/qis/vcpkg-test/blob/2d0693df8f04930f0933daaceef0519f490d1db9/giflib/src/main.cpp#L16