daemon: Always default to gzip for log compression.

* nix/libstore/globals.cc (Settings::Settings): Have 'logCompression'
default to COMPRESSION_GZIP unconditionally.
* gnu/services/base.scm (<guix-configuration>)[log-compression]: Default
to 'gzip.
* doc/guix.texi (Invoking guix-daemon, Base Services): Adjust accordingly.

doc/guix.texi

@@ -1705,7 +1705,7 @@ Compress build logs according to @var{type}, one of @code{gzip},
 
 Unless @option{--lose-logs} is used, all the build logs are kept in the
 @var{localstatedir}.  To save space, the daemon automatically compresses
-them with Bzip2 by default.
+them with gzip by default.
 
 @item --discover[=yes|no]
 Whether to discover substitute servers on the local network using mDNS
@@ -16623,7 +16623,7 @@ The number of seconds of silence and the number of seconds of activity,
 respectively, after which a build process times out.  A value of zero
 disables the timeout.
 
-@item @code{log-compression} (default: @code{'bzip2})
+@item @code{log-compression} (default: @code{'gzip})
 The type of compression used for build logs---one of @code{gzip},
 @code{bzip2}, or @code{none}.
 

gnu/services/base.scm

@@ -1566,7 +1566,7 @@ (define-record-type* <guix-configuration>
   (timeout          guix-configuration-timeout    ;integer
                     (default 0))
   (log-compression  guix-configuration-log-compression
-                    (default 'bzip2))
+                    (default 'gzip))
   (discover?        guix-configuration-discover?
                     (default #f))
   (extra-options    guix-configuration-extra-options ;list of strings

nix/libstore/globals.cc

@@ -45,11 +45,7 @@ Settings::Settings()
     useChroot = false;
     impersonateLinux26 = false;
     keepLog = true;
-#if HAVE_BZLIB_H
-    logCompression = COMPRESSION_BZIP2;
-#else
     logCompression = COMPRESSION_GZIP;
-#endif
     maxLogSize = 0;
     cacheFailure = false;
     pollInterval = 5;