docs: Windows link notes apply to MinGW too, not only Visual Studio

The section on shlwapi and BENCHMARK_STATIC_DEFINE was titled for Visual
Studio only, so MinGW-w64 users following the README's g++ command hit
undefined __imp_ references without finding the fix. Retitle it
"Building on Windows", state the two requirements up front, add the
MinGW-w64 command line, and point to it from the README.

Fixes google#2309

Author: devtejasx

README.md

@@ -193,6 +193,10 @@ $ g++ mybenchmark.cc -std=c++11 -isystem benchmark/include \
   -Lbenchmark/build/src -lbenchmark -lpthread -o mybenchmark
 ```
 
+On Windows, including MinGW-w64, this also needs `-DBENCHMARK_STATIC_DEFINE`
+and `-lshlwapi`; see
+[Building on Windows](docs/platform_specific_build_instructions.md#building-on-windows).
+
 Alternatively, link against the `benchmark_main` library and remove
 `BENCHMARK_MAIN();` above to get the same behavior.
 

docs/platform_specific_build_instructions.md

@@ -15,9 +15,30 @@ On QNX, the pthread library is part of libc and usually included automatically
 [`pthread_create()`](https://www.qnx.com/developers/docs/7.1/index.html#com.qnx.doc.neutrino.lib_ref/topic/p/pthread_create.html)).
 There's no separate pthread library to link.
 
-## Building with Visual Studio 2015, 2017 or 2022
+## Building on Windows
 
-The `shlwapi` library (`-lshlwapi`) is required to support a call to `CPUInfo` which reads the registry. Either add `shlwapi.lib` under `[ Configuration Properties > Linker > Input ]`, or use the following:
+The notes in this section apply to every Windows toolchain, not only to Visual
+Studio:
+
+* The `shlwapi` library is required to support a call to `CPUInfo` which reads
+  the registry.
+* When linking against the static library without going through CMake's
+  `benchmark::benchmark` target, `BENCHMARK_STATIC_DEFINE` must be defined.
+  Without it the headers declare every symbol as imported from a DLL, and the
+  link fails with `undefined reference to __imp_...` errors.
+
+### MinGW-w64 (MSYS2) and Clang
+
+Add both to the compile command from the README:
+
+```
+$ g++ mybenchmark.cc -std=c++17 -DBENCHMARK_STATIC_DEFINE -isystem benchmark/include \
+  -Lbenchmark/build/src -lbenchmark -lpthread -lshlwapi -o mybenchmark
+```
+
+### Visual Studio 2015, 2017 or 2022
+
+Either add `shlwapi.lib` under `[ Configuration Properties > Linker > Input ]`, or use the following:
 
 ```
 // Alternatively, can add libraries using linker options.