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 ()
ree/tests/services/lightdm.scm?id=1f96da1f49e847fc3b001ae3bfccaba92a08fc6d'>tree</a><a href='/guix/commit/tests/services/lightdm.scm?id=1f96da1f49e847fc3b001ae3bfccaba92a08fc6d'>commit</a><a href='/guix/diff/tests/services/lightdm.scm?id=1f96da1f49e847fc3b001ae3bfccaba92a08fc6d'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/tests/services/lightdm.scm'>
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<div class='path'>path: <a href='/guix/log/?id=1f96da1f49e847fc3b001ae3bfccaba92a08fc6d'>root</a>/<a href='/guix/log/tests?id=1f96da1f49e847fc3b001ae3bfccaba92a08fc6d'>tests</a>/<a href='/guix/log/tests/services?id=1f96da1f49e847fc3b001ae3bfccaba92a08fc6d'>services</a>/<a href='/guix/log/tests/services/lightdm.scm?id=1f96da1f49e847fc3b001ae3bfccaba92a08fc6d'>lightdm.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/services/lightdm.scm?id=1f96da1f49e847fc3b001ae3bfccaba92a08fc6d&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-08-28 22:28:47 -0400'>2022-08-28</span></td><td><a href='/guix/commit/tests/services/lightdm.scm?id=0ea62e84a787fe94cfeadf67ef27ea995a382b84'>services: Add lightdm-service-type.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/lightdm.scm: New service.
* tests/services/lightdm.scm: Test it.
* doc/guix.texi (X Window): Document it.
* gnu/local.mk (GNU_SYSTEM_MODULES): Register it.

Co-authored-by: L p R n d n &lt;guix@lprndn.info&gt;
Co-authored-by: Ricardo Wurmus &lt;rekado@elephly.net&gt;
</span></span></td><td>Maxim Cournoyer</td></tr>