Fix required by suitesparse to build Mongoose

The CMakeLists.txt of Mongoose assumes that SuiteSparse_config has been
installed into the suitesparse source directory, which is not the case
for us, as we are building suitesparse out-of-tree.

SuiteSparse_config can instead be found in the ${CMAKE_INSTALL_PREFIX}
directory.

diff --git a/Mongoose/CMakeLists.txt b/Mongoose/CMakeLists.txt
index 7e134ab..76fa9e2 100644
--- a/Mongoose/CMakeLists.txt
+++ b/Mongoose/CMakeLists.txt
@@ -148,10 +148,10 @@ set(CMAKE_CXX_STANDARD 11)
 #set(CMAKE_CXX_STANDARD_REQUIRED ON)

 # determine which SuiteSparse_config to use
-if (EXISTS ${PROJECT_SOURCE_DIR}/../SuiteSparse_config)
-    message(STATUS "External ../SuiteSparse_config" ${BoldBlue} " found" ${ColourReset} ".")
+if (EXISTS ${CMAKE_INSTALL_PREFIX})
+    message(STATUS "External SuiteSparse_config" ${BoldBlue} " found" ${ColourReset} ".")
     set ( SUITESPARSE_CONFIG_DIR ${PROJECT_SOURCE_DIR}/../SuiteSparse_config )
-    link_directories ( ${PROJECT_SOURCE_DIR}/../lib )
+    link_directories ( ${CMAKE_INSTALL_PREFIX}/lib )
     message ( STATUS "Note: ../SuiteSparse_config must be compiled before compiling Mongoose" )
     set ( SUITESPARSE_CONFIG_LIBRARY suitesparseconfig )
 else ()
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<tr><td><span title='2021-08-02 17:23:30 -0400'>2021-08-02</span></td><td><a href='/guix/commit/gnu/tests/data?id=10f554700c417b84c22c56fdd007567a52c45d75'>Reinstate "services: Add a service for Jami."</a><span class='msg-avail'>...<span class='msg-tooltip'>This reverts commit 4673f817938d9d2b1b40a072ab2e0c44a32ccc97, which reverted
commit 69dcc24c9f0cdfea674eb690e7755d26a25ced2b with the fix detailed below.

Thanks to Christopher Baines for reporting the failure and proposing a fix.

* guix/self.scm (compiled-guix) [*system-test-modules*]: Add the test data
files via the 'extra-files' argument.
* gnu/local.mk (dist_patch_DATA): Move the tests/data/jami-dummy-account.dat
file to...
* gnu/local.mk (MODULES_NOT_COMPILED): ... here.
</span></span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2021-08-02 15:48:59 -0400'>2021-08-02</span></td><td><a href='/guix/commit/gnu/tests/data?id=4673f817938d9d2b1b40a072ab2e0c44a32ccc97'>Revert "services: Add a service for Jami."</a><span class='msg-avail'>...<span class='msg-tooltip'>This reverts commit 69dcc24c9f0cdfea674eb690e7755d26a25ced2b.  It broke 'guix
pull'.
</span></span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2021-08-02 15:15:02 -0400'>2021-08-02</span></td><td><a href='/guix/commit/gnu/tests/data?id=69dcc24c9f0cdfea674eb690e7755d26a25ced2b'>services: Add a service for Jami.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/telephony.scm (string-or-computed-file?)
(string-list?, account-fingerprint-list?): New procedures.
(maybe-string-list, maybe-account-fingerprint-list)
(maybe-boolean, maybe-string, jami-account-list): New configuration field
types.
(serialize-string-list, serialize-boolean, serialize-string)
(jami-account, jami-account-&gt;alist, jami-configuration)
(jami-account-list?, jami-account-list-maybe): New procedures.
(%jami-accounts): New variable.
(jami-configuration-&gt;command-line-arguments): New procedure.
(jami-dbus-session-activation, jami-shepherd-services): New procedures.
(jami-service-type): New variable.
* gnu/build/jami-service.scm: New file.
* gnu/tests/data/jami-dummy-account.dat: Likewise.
* gnu/tests/telephony.scm: Likewise.
* gnu/local.mk (GNU_SYSTEM_MODULES): Register them.
* Makefile.am (SCM_TESTS): Register the test file.
(dist_patch_DATA): Register the new data file.
* doc/guix.texi (Telephony Services): Document it.
</span></span></td><td>Maxim Cournoyer</td></tr>