nix/tests: run test help.sh only if nix is built with documentation

tests/functional/help.sh calls nix-* commands with option --help
if nix is built without documentation the option --help throws an error
because the man page it wants to display is missing

configure.ac

@@ -170,6 +170,10 @@ AS_IF(
   [test "$ENABLE_FUNCTIONAL_TESTS" == "yes" || test "$ENABLE_DOC_GEN" == "yes"],
   [NEED_PROG(jq, jq)])
 
+AS_IF(
+  [test "$ENABLE_DOC_GEN" == "yes"],
+  [NEED_PROG(man, man)])
+
 AS_IF([test "$ENABLE_BUILD" == "yes"],[
 
 # Look for boost, a required dependency.

package.nix

@@ -207,9 +207,10 @@ in {
     git
     mercurial
     openssh
-    man # for testing `nix-* --help`
   ] ++ lib.optionals (doInstallCheck || enableManual) [
     jq # Also for custom mdBook preprocessor.
+  ] ++ lib.optionals enableManual [
+    man
   ] ++ lib.optional stdenv.hostPlatform.isStatic unixtools.hexdump
   ;
 

tests/functional/local.mk

@@ -114,7 +114,6 @@ nix_tests = \
   impure-env.sh \
   debugger.sh \
   extra-sandbox-profile.sh \
-  help.sh
 
 ifeq ($(HAVE_LIBCPUID), 1)
   nix_tests += compute-levels.sh
@@ -128,6 +127,10 @@ ifeq ($(ENABLE_BUILD), yes)
   endif
 endif
 
+ifeq ($(ENABLE_DOC_GEN), yes)
+    nix_tests += help.sh
+endif
+
 $(d)/test-libstoreconsumer.sh.test $(d)/test-libstoreconsumer.sh.test-debug: \
   $(buildprefix)$(d)/test-libstoreconsumer/test-libstoreconsumer
 $(d)/plugins.sh.test $(d)/plugins.sh.test-debug: \