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 ()
='/guix/tree/tests/store-deduplication.scm?id=2df940ab273a79311e1a7275fb5d491c96b4115f'>tree</a><a href='/guix/commit/tests/store-deduplication.scm?id=2df940ab273a79311e1a7275fb5d491c96b4115f'>commit</a><a href='/guix/diff/tests/store-deduplication.scm?id=2df940ab273a79311e1a7275fb5d491c96b4115f'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/tests/store-deduplication.scm'>
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<div class='path'>path: <a href='/guix/log/?id=2df940ab273a79311e1a7275fb5d491c96b4115f'>root</a>/<a href='/guix/log/tests?id=2df940ab273a79311e1a7275fb5d491c96b4115f'>tests</a>/<a href='/guix/log/tests/store-deduplication.scm?id=2df940ab273a79311e1a7275fb5d491c96b4115f'>store-deduplication.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/tests/store-deduplication.scm?id=2df940ab273a79311e1a7275fb5d491c96b4115f&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-12-10 10:56:48 +0100'>2022-12-10</span></td><td><a href='/guix/commit/tests/store-deduplication.scm?id=b129026e2e242e9068158ae6e6fcd8d7c5ea092e'>deduplicate: Use 'sendfile' for small file copies.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/store/deduplication.scm (dump-file/deduplicate): Use 'sendfile'
instead of 'dump-port'.
* tests/store-deduplication.scm ("copy-file/deduplicate, below %deduplication-minimum-size"):
New test.
</span></span></td><td>Ludovic Courtès</td></tr>