add doc/man to subdir if configure flag --enable-man is set

Makefile.am

@@ -1,5 +1,8 @@
 ACLOCAL_AMFLAGS = -I build-aux/m4
 SUBDIRS = src
+if ENABLE_MAN
+SUBDIRS += doc/man
+endif
 .PHONY: deploy FORCE
 
 GZIP_ENV="-9n"
@@ -213,6 +216,8 @@ DISTCLEANFILES = qa/pull-tester/tests_config.pyc
 
 .INTERMEDIATE: $(COVERAGE_INFO)
 
+DISTCHECK_CONFIGURE_FLAGS = --enable-man
+
 clean-local:
 	rm -rf coverage_percent.txt test_bitcoin.coverage/ total.coverage/ qa/tmp/ cache/ $(OSX_APP)
 	rm -rf qa/pull-tester/__pycache__