restart flang migration; add commit msg

recipe/migrations/flang19.yaml

@@ -1,8 +1,33 @@
 __migrator:
   kind: version
-  migration_number: 1
+  migration_number: 2
   build_number: 1
-  commit_message: Rebuild for flang 19
+  commit_message: |
+    Rebuild for flang 19
+    
+    TL;DR: We are trying to switch our Fortran compilers on windows to flang.
+    This is not 100% guaranteed to work, but should be fine in the majority of cases.
+    
+    The new LLVM-based flang has become mature enough that it should be possible to
+    broadly switch over our Fortran compilers on windows to it (until now we only had
+    an ancient pre-LLVM flang 5, or alternatively the GCC-based `m2w64_fortran`).
+    
+    As such, this PR attempts to homogenize any use of `m2w64_fortran` and other `m2w64_*`
+    compilers to our default stack (which would then be MSVC + flang on windows), with
+    the exception of feedstocks for R-packages, which stay on the `m2w64_` compilers.
+    
+    Recipes that have hard-coded expectations about the name of the fortran compiler
+    will need to adjust to use `%FC%` or `flang-new` for the compiler name. Similarly,
+    you may need to change the linker to `%LD%` or use `lld-link`.
+    
+    It is also possible that you run into compilation errors due to differences in
+    compiler behaviour, bugs or as-yet unimplemented features. In case of compilation
+    errors due to stricter default language standards, you should be able to fix things
+    by passing `-std=legacy` to `FFLAGS`.
+    
+    If you have problems with this PR, feel free to ping the @c-f/flang-activation team.
+    In case you have convinced yourself that flang really is not ready yet to be used to
+    compile a given feedstock, you may also close this migrator PR.
   platform_allowlist:
     - win-64
   override_cbc_keys:
@@ -92,6 +117,7 @@ __migrator:
     - r-lokern
     - r-lsei
     - r-mclust
+    - r-mcr
     - r-mda
     - r-mets
     - r-minpack.lm