profiles: Build the man database only if 'man-db' is in the profile.

This allows us to skip the expensive man-db profile hook in most cases.

Suggested by Liliana Marie Prikler <liliana.prikler@gmail.com>.

* guix/profiles.scm (manual-database/optional): New procedure.
(%default-profile-hooks): Use it instead of 'manual-database'.
* doc/guix.texi (Documentation): Add footnote about 'man -k' database
creation.

doc/guix.texi

@@ -36611,7 +36611,9 @@ $ info -k TLS
 @end example
 
 @noindent
-The command below searches for the same keyword in man pages:
+The command below searches for the same keyword in man
+pages@footnote{The database searched by @command{man -k} is only created
+in profiles that contain the @code{man-db} package.}:
 
 @example
 $ man -k TLS

guix/profiles.scm

@@ -1682,6 +1682,16 @@ the entries in MANIFEST."
                     `((type . profile-hook)
                       (hook . manual-database))))
 
+(define (manual-database/optional manifest)
+  "Return a derivation to build the manual database of MANIFEST, but only if
+MANIFEST contains the \"man-db\" package.  Otherwise, return #f."
+  ;; Building the man database (for "man -k") is expensive and rarely used.
+  ;; Build it only if the profile also contains "man-db".
+  (mlet %store-monad ((man-db (manifest-lookup-package manifest "man-db")))
+    (if man-db
+        (manual-database manifest)
+        (return #f))))
+
 (define (texlive-configuration manifest)
   "Return a derivation that builds a TeXlive configuration for the entries in
 MANIFEST."
@@ -1784,7 +1794,7 @@ MANIFEST."
   ;; This is the list of derivation-returning procedures that are called by
   ;; default when making a non-empty profile.
   (list info-dir-file
-        manual-database
+        manual-database/optional
         fonts-dir-file
         ghc-package-cache-file
         ca-certificate-bundle