Updated CMake configuration

[wipe2238]

• Requires newer CMake version with more target-focused configuraton
  Minimum required version set to 3.18.4, released October 2020; available on Debian 11 (oldstable)
• Splits current configuration into static library (int2ssl-lib) and executable (int2ssl) targets
  Different approach to integrating CFalloutScript into external application; instead of messing with code first, i'm messing with build process. That makes integration available faster as long external application provides everything required for linking.
• Provides a way to test if int2ssl-lib is truly independent of int2ssl
  Added int2ssl-lib-test target (not built by default) which tries to use library target with absolute minimum setup required. Generated only for out-of-source builds.
• Provides a way to add compiler/linker options for different platforms/setups without splitting them into wall of ifs
  Added helper function int2ssl_build_option() which allows...
  - selecting if flag should be added to compiler or linker
  - filtering out flags which are irrelevant (disabled options, different platforms, etc.)
  - marking flag as required
  ...all in single line, making flags easier to read/find/add/remove, as they're all in one place
• Allows disabling building executable target
  Added option INT2SSL_BINARY (default: on)
• Enables more compiler warnings, with option to promote them to errors (linux)
  Added compiler flags -Wall -Wextra -Wpedantic
  Added option INT2SSL_STRICT (default: off) which turns on compiler flag -Werror

Extras

• Includes very basic info about int2ssl in project() command
  If someone includes int2ssl in their application source, finding this repository will be one click away :)
• Allows building static executable (linux)
  Added option INT2SSL_BINARY_STATIC (default: off)
  Required compiler flag(s) are currently checked in if(UNIX) block only, simply because i'm not using MinGW right now, and have no way to test all little details. For crossplatform compilation it should probably be changed to something like if(INT2SSL_BINARY_STATIC AND CMAKE_CXX_COMPILER_ID MATCHES "GNU") or something like that, but someone needs to test it first~
• Adds out-of-source build directory to git ignore list
  Clean worktree is happy worktree