environments: more info re ucrt/llvm etc

Just so we have "something" to link to. Needs more work.

Author: lazka

Diff for: web/docs/environments.md

@@ -30,3 +30,48 @@ tools.
 | ![clang64](clang64.png){: style="max-width:25px" } | **CLANG64** | `/clang64` | llvm | x86_64 | ucrt | libc++ |
 | ![mingw32](mingw32.png){: style="max-width:25px" } | **MINGW32** | `/mingw32` |  gcc | i686  | msvcrt | libstdc++ |
 
+## GCC vs LLVM/Clang
+
+These are the default compilers/toolchains used for building all packages in the
+respective repositories.
+
+**GCC** based environments:
+
+* Widely tested/used at this point
+* Fortran support
+* While there also exists a Clang package in the MINGW environments, that one
+  still uses the GNU linker and the GNU C++ library. In same cases Clang is used
+  to build packages as well there, in case upstream prefers Clang over gcc for
+  example.
+
+**LLVM/Clang** based environments:
+
+* Only uses LLVM tools, LLD as a linker, LIBC++ as a C++ standard library
+* Clang provides ASAN support
+* Native TLS support
+* LLD is faster than LD, but does not support all the features LD supports
+* Some tools lack feature parity with equivalent GNU tools
+* Supports ARM64 on Windows
+
+## MSVCRT vs UCRT
+
+These are two variants of the C standard library on Windows.
+
+**MSVCRT** is available by default on all Windows versions, but due to backwards
+compatibility issues is stuck in the past, not C99 compatible and is missing
+some features.
+
+* It isn't C99 compatible, for example the printf() function family, but...
+* mingw-w64 provides replacement functions to make things C99 compatible in many
+  cases
+* It doesn't support the UTF-8 locale
+* Because MSVC uses UCRT, and CRTs shouldn't be mixed, you can't mix it with
+  libraries built with MSVC.
+* Works out of the box on every Windows machine.
+
+**UCRT** is a newer version which is also used by Visual Studio by default. It
+should work and behave as if the code was compiled with MSVC.
+
+* Better compatibility with MSVC, both at build time and at run time.
+* It only ships by default on Windows 10 and for older versions you have to
+  provide it yourself or depend on the user having it installed.