gnu: hypre: Fix documentation.

* gnu/packages/maths.scm (hypre)[native-inputs]: Add ghostscript.
[arguments]<phases>{build-docs}: Set $HOME so that fontconfig finds a
directory for writing its cache.

Change-Id: I46635f262a0d3ecd4114d1d673da635232522915
Co-authored-by: Andreas Enge <andreas@enge.fr>
Signed-off-by: Andreas Enge <andreas@enge.fr>

gnu/packages/maths.scm

@@ -7752,6 +7752,7 @@ set.")
     (native-inputs
      (list doc++
            doxygen
+           ghostscript
            python
            python-breathe
            python-sphinx
@@ -7824,6 +7825,7 @@ set.")
                                                 configure-flags)))))))
                (add-after 'build 'build-docs
                  (lambda _
+                   (setenv "HOME" (getcwd))
                    (invoke "make" "-C" "docs")))
                (replace 'check
                  (lambda* (#:key tests? #:allow-other-keys)