- Fixed scanf in IWAD picker without GTK

If you try to kill the program with Ctrl-C, it would run the first IWAD available in the list instead of closing it.
- Remove clang check from GCC-related workaround

It caused a clang warning/error regarding the non-existing flags '-fno-tree-dominator-opts' and '-fno-tree-fre'.
- Fixed overflow checking in some viewpitch code
Merge branch 'cmake_clang_flags_fix' of https://github.com/edward-san/zdoom into maint
Merge branch 'viewpitch_overflow_fix' of https://github.com/edward-san/zdoom into maint
Merge branch 'maint'