gnu: guile-ssh: Support cross-compilation.

Cross-compilation broke with the update to 0.15.1 in
eabc6af9b8.  'guile-snarf' would invoke
cpp instead of TRIPLET-cpp, which would thus fail to find libssh
headers.

* gnu/packages/ssh.scm (guile-ssh)[arguments]: Add
'support-cross-compilation' phase.

gnu/packages/ssh.scm

@@ -328,6 +328,13 @@ (define-public guile-ssh
        #:configure-flags '("--disable-static")
 
        #:phases (modify-phases %standard-phases
+                  (add-before 'bootstrap 'support-cross-compilation
+                    (lambda _
+                      ;; Support cross-compilation:
+                      ;; <https://github.com/artyom-poptsov/guile-ssh/issues/30>.
+                      (substitute* "libguile-ssh/Makefile.am"
+                        (("\\$\\(guile_snarf\\)")
+                         "CPP=\"$(CPP)\" $(guile_snarf)"))))
                   (add-before 'build 'fix-libguile-ssh-file-name
                     (lambda* (#:key outputs #:allow-other-keys)
                       ;; Build and install libguile-ssh.so so that we can use