Build system: add mk/validate.mk.sample

Reviewers: austin, bgamari

Subscribers: erikd

Differential Revision: https://phabricator.haskell.org/D1303

	modified:   configure.ac

configure.ac

@@ -1197,8 +1197,8 @@ echo ["\
 fi
 
 echo ["\
-   Building Sphinx HTML documentation : $BUILD_SPHINX_HTML
-   Building Sphinx PDF documentation  : $BUILD_SPHINX_PDF"]
+   Tools to build Sphinx HTML documentation available: $BUILD_SPHINX_HTML
+   Tools to build Sphinx PDF documentation available: $BUILD_SPHINX_PDF"]
 
 echo ["----------------------------------------------------------------------
 "]

ghc.mk

@@ -171,7 +171,8 @@ endif
 ifeq "$(BUILD_SPHINX_HTML)" "YES"
 ifeq "$(SPHINXBUILD)" ""
 $(error BUILD_SPHINX_HTML=YES, but `sphinx-build` was not found. \
-  Install `sphinx-build`, then rerun `./configure`. \
+  Create a file `mk/validate.mk` containing `BUILD_SPHINX_HTML=NO` \
+  (when validating), or install `sphinx-build` and rerun `./configure`. \
   See https://ghc.haskell.org/trac/ghc/wiki/Building/Preparation)
 endif
 endif

mk/validate.mk.sample

+# These settings override mk/flavours/validate.mk (see
+# mk/custom-settings.mk).
+#HADDOCK_DOCS       = NO
+#BUILD_SPHINX_HTML  = NO