emacs: add two parameters to genericBuild to control errors

This patch introduces two parameters, turnCompilationWarningToError
and ignoreCompilationError, to control errors in genericBuild, which
makes "nix build" be able to fail at elisp native compilation errors
or warnings.  This feature can be used in CI to improve code quality.

Note that this patch keeps the old behavior by default.  Hopefully one
day we can flip the default value of ignoreCompilationError to false
when enough packages are fixed.

Also note that these two parameters can be changed per package using
the overrideAttrs interface.

pkgs/applications/editors/emacs/build-support/generic.nix

@@ -27,6 +27,8 @@ in
 , buildInputs ? []
 , packageRequires ? []
 , meta ? {}
+, turnCompilationWarningToError ? false
+, ignoreCompilationError ? true
 , ...
 }@args:
 
@@ -73,6 +75,8 @@ stdenv.mkDerivation (finalAttrs: ({
 
   addEmacsNativeLoadPath = true;
 
+  inherit turnCompilationWarningToError ignoreCompilationError;
+
   postInstall = ''
     # Besides adding the output directory to the native load path, make sure
     # the current package's elisp files are in the load path, otherwise
@@ -86,8 +90,9 @@ stdenv.mkDerivation (finalAttrs: ({
           "emacs \
              --batch \
              --eval '(setq large-file-warning-threshold nil)' \
+             --eval '(setq byte-compile-error-on-warn ${if finalAttrs.turnCompilationWarningToError then "t" else "nil"})' \
              -f batch-native-compile {} \
-           || true"
+           || exit ${if finalAttrs.ignoreCompilationError then "0" else "\\$?"}"
   '';
 }