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 ()
ref='/guix/tree/tests/guix-git-authenticate.sh?id=86825f7f85f91ff648416fea2198d8d36ea54c7f'>tree</a><a href='/guix/commit/tests/guix-git-authenticate.sh?id=86825f7f85f91ff648416fea2198d8d36ea54c7f'>commit</a><a href='/guix/diff/tests/guix-git-authenticate.sh?id=86825f7f85f91ff648416fea2198d8d36ea54c7f'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/tests/guix-git-authenticate.sh'>
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<div class='path'>path: <a href='/guix/log/?id=86825f7f85f91ff648416fea2198d8d36ea54c7f'>root</a>/<a href='/guix/log/tests?id=86825f7f85f91ff648416fea2198d8d36ea54c7f'>tests</a>/<a href='/guix/log/tests/guix-git-authenticate.sh?id=86825f7f85f91ff648416fea2198d8d36ea54c7f'>guix-git-authenticate.sh</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/guix-git-authenticate.sh?id=86825f7f85f91ff648416fea2198d8d36ea54c7f&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-09-28 21:06:36 -0500'>2020-09-28</span></td><td><a href='/guix/commit/tests/guix-git-authenticate.sh?id=d8934360d2453a403b5433e71d09188e4ed23b57'>tests: Simplify shell exit status negation;</a><span class='msg-avail'>...<span class='msg-tooltip'>* tests/guix-archive.sh, tests/guix-build-branch.sh, tests/guix-build.sh,
tests/guix-daemon.sh, tests/guix-download.sh, tests/guix-environment.sh,
tests/guix-gc.sh, tests/guix-git-authenticate.sh, tests/guix-graph.sh,
tests/guix-hash.sh, tests/guix-lint.sh, tests/guix-pack-relocatable.sh,
tests/guix-pack.sh, tests/guix-package-aliases.sh, tests/guix-package-net.sh,
tests/guix-package.sh: Use the shell '!' keyword to negate command exit status
in place of 'if ...; then false; else true; fi'
</span></span></td><td>Eric Bavier</td></tr>
<tr><td><span title='2020-07-11 12:33:01 +0200'>2020-07-11</span></td><td><a href='/guix/commit/tests/guix-git-authenticate.sh?id=a98712785e0b042a290420fd74e5a4a5da4fc68f'>Add 'guix git authenticate'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/scripts/git.scm, guix/scripts/git/authenticate.scm,
tests/guix-git-authenticate.sh: New files.
* Makefile.am (MODULES): Add the *.scm files.
(SH_TESTS): Add 'tests/guix-git-authenticate.sh'.
* doc/guix.texi (Channels)[Specifying Channel Authorizations]: Mention
'guix git authenticate'.
(Invoking guix git authenticate): New node.
* po/guix/POTFILES.in: Add 'guix/scripts/git.scm' and
'guix/scripts/git/authenticate.scm'.
</span></span></td><td>Ludovic Courtès</td></tr>