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 ()
s/dict.scm?id=6e1a88756c2958b33cdb7a010d195bdc426ee896'>tree</a><a href='/guix/commit/gnu/services/dict.scm?id=6e1a88756c2958b33cdb7a010d195bdc426ee896'>commit</a><a href='/guix/diff/gnu/services/dict.scm?id=6e1a88756c2958b33cdb7a010d195bdc426ee896'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/services/dict.scm'>
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<div class='path'>path: <a href='/guix/log/?id=6e1a88756c2958b33cdb7a010d195bdc426ee896'>root</a>/<a href='/guix/log/gnu?id=6e1a88756c2958b33cdb7a010d195bdc426ee896'>gnu</a>/<a href='/guix/log/gnu/services?id=6e1a88756c2958b33cdb7a010d195bdc426ee896'>services</a>/<a href='/guix/log/gnu/services/dict.scm?id=6e1a88756c2958b33cdb7a010d195bdc426ee896'>dict.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/gnu/services/dict.scm?id=6e1a88756c2958b33cdb7a010d195bdc426ee896&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-09-21 00:31:15 +0200'>2020-09-21</span></td><td><a href='/guix/commit/gnu/services/dict.scm?id=b864ddb6905a1c4e3a88a6e8214b58f42ff5bf19'>services: dicod: Reduce irony.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/dict.scm (dicod-service): Fix docstring typo.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-05-08 16:30:50 +0200'>2020-05-08</span></td><td><a href='/guix/commit/gnu/services/dict.scm?id=dd0804c67e99914eecc471fdb92f4220425a1cbc'>services: Add descriptions.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/dbus.scm (polkit-service-type)[description]: New field.
* gnu/services/dict.scm (dicod-service-type)[description]: New field.
* gnu/services/dns.scm (knot-service-type)[description]: New field.
* gnu/services/networking.scm (dhcpd-service-type)[description]: New field.
* gnu/services/shepherd.scm (shepherd-root-service-type)[description]:
New field.
* gnu/services/xorg.scm (slim-service-type)[description]: New field.
(screen-locker-service-type)[description]: New field.
* gnu/system/pam.scm (pam-root-service-type)[description]: New field.
* gnu/system/shadow.scm (account-service-type)[description]: New field.
</span></span></td><td>Ludovic Courtès</td></tr>