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='2023-02-25 23:10:59 -0500'>2023-02-25</span></td><td><a href='/guix/commit/.patman?id=cb0d8100b288b5b0d130820207db17764b7d2140'>.patman: Ignore bad tags.</a><span class='msg-avail'>...<span class='msg-tooltip'>The feature is not used in Guix, and it produces extraneous warnings.

* .patman (ignore_bad_tags): Set option to True.
</span></span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2022-12-28 15:36:33 -0500'>2022-12-28</span></td><td><a href='/guix/commit/.patman?id=fde5af3962c8fafc5d20e5d742cc7aea907f3563'>.patman: New configuration file.</a><span class='msg-avail'>...<span class='msg-tooltip'>* .patman: New file.

Series-to: 60218@debbugs.gnu.org
Series-cc: rekado@elephly.net, othacehe@gnu.org
Series-version: 2
</span></span></td><td>Maxim Cournoyer</td></tr>