Don't output manpage in same directory as source

Test Plan: Validate

Reviewers: austin, thomie, nomeata

Differential Revision: https://phabricator.haskell.org/D1782

GHC Trac Issues: #11433

.gitignore

@@ -98,6 +98,7 @@ _darcs/
 /docs/users_guide/users_guide.pdf
 /docs/users_guide/build-html
 /docs/users_guide/build-pdf
+/docs/users_guide/build-man
 /docs/users_guide/.doctrees-*
 /docs/users_guide/.doctrees/
 /driver/ghci/ghc-pkg-inplace

docs/users_guide/ghc.mk

@@ -23,11 +23,11 @@ html_docs/users_guide : docs/users_guide/images/prof_scc.svg
 docs/users_guide_MAN_RST_SOURCES := docs/users_guide/ghc.rst
 
 MAN_SECTION := 1
-MAN_PAGES := docs/users_guide/ghc.1
+MAN_PAGES := docs/users_guide/build-man/ghc.1
 
 ifneq "$(BINDIST)" "YES"
 $(MAN_PAGES): $(docs/users_guide_MAN_RST_SOURCES) $(utils/mkUserGuidePart_GENERATED_RST_SOURCES)
-	$(SPHINXBUILD) -b man -d docs/users_guide/.doctrees-man docs/users_guide docs/users_guide
+	$(SPHINXBUILD) -b man -d docs/users_guide/.doctrees-man docs/users_guide docs/users_guide/build-man
 endif
 
 man : $(MAN_PAGES)