Create a "build systems" section

The goal is to describe how to use them to build/cross compile,
how to detect mingw if needed, and point out differences to MSVC,
maybe even differences to Linux while at it.

mkdocs.yml

@@ -70,6 +70,8 @@ nav:
     - getting-started/debian.md
     - getting-started/archlinux.md
     - getting-started/msys2.md
+  - Build Systems:
+    - build-systems/meson.md
   - contribute.md
   - support.md
   - donate.md

web/build-systems/meson.md

@@ -0,0 +1,59 @@
+# Meson
+
+## Detect mingw-w64 at Configuration Time
+
+```meson
+# meson.build
+cc = meson.get_compiler('c')
+is_windows = host_machine.system() == 'windows'
+is_mingw = is_windows and cc.get_define('__MINGW32__') != ''
+if is_mingw
+    message('Building with mingw-w64')
+else
+    message('Not building with mingw-w64')
+endif
+```
+
+## Cross compiling on Linux
+
+```c
+// hello.c
+#include <stdio.h>
+
+int main(void) {
+    printf("Hello, Windows!\n");
+    return 0;
+}
+```
+
+```ini
+# cross-mingw64.txt
+[host_machine]
+system = 'windows'
+cpu_family = 'x86_64'
+cpu = 'x86_64'
+endian = 'little'
+
+[binaries]
+c = 'x86_64-w64-mingw32-gcc'
+cpp = 'x86_64-w64-mingw32-g++'
+ar = 'x86_64-w64-mingw32-ar'
+ld = 'x86_64-w64-mingw32-ld'
+objcopy = 'x86_64-w64-mingw32-objcopy'
+strip = 'x86_64-w64-mingw32-strip'
+pkg-config = 'x86_64-w64-mingw32-pkg-config'
+windres = 'x86_64-w64-mingw32-windres'
+```
+
+```meson
+# meson.build
+project('hello', 'c')
+executable('hello', 'hello.c')
+```
+
+```console
+$ meson setup builddir --cross-file cross-mingw64.txt
+$ meson compile -C builddir
+$ wine builddir/hello.exe
+Hello, Windows!
+```