gh-38934: keep meson.build file for ext/interpreters
    
`make sagelib-clean` should not remove any `meson.build` files, but now
it does, the one in
`src/sage/ext/interpreters/`.
As  reported here: https://groups.google.com/g/sage-
release/c/HHbr1W7vDoM/m/qLYBuIcpCAAJ
This PR fixes this.

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [ x] The title is concise and informative.
- [x ] The description explains in detail what this PR is about.
- [ x] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - #12345: short description why this is a dependency -->
<!-- - #34567: ... -->
    
URL: #38934
Reported by: Dima Pasechnik
Reviewer(s): Dima Pasechnik, Gonzalo Tornaría, Tobias Diez

Author: Release Manager

.gitignore

@@ -150,6 +150,7 @@ __pycache__/
 # Generated by sage_setup.autogen
 /src/sage/ext/interpreters
 !/src/sage/ext/interpreters/meson.build
+!/src/sage/ext/interpreters/README
 
 # Generated Cython files
 *.so

Makefile

@@ -122,7 +122,7 @@ sagelib-clean:
 	     rm -rf c_lib .cython_version cython_debug; \
 	     rm -rf build; find . -name '*.pyc' -o -name "*.so" | xargs rm -f; \
 	     rm -f $$(find . -name "*.pyx" | sed 's/\(.*\)[.]pyx$$/\1.c \1.cpp/'); \
-	     rm -rf sage/ext/interpreters) \
+	     cd sage/ext/interpreters/ && rm -f *.so *.c *.h *.py* *.pxd) \
 	    && (cd "$(SAGE_ROOT)/build/pkgs/sagelib/src/" && rm -rf build); \
 	fi
 

src/sage/ext/interpreters/README

@@ -0,0 +1 @@
+The files in this directory, except meson.build, are autogenerated by sage_setup.autogen