gh-37161: Fix cvxopt build with suitesparse homebrew; fixes #37149.

    
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This PR tries to fix issue #37149.

The problem is that cvxopt doesn't build correctly when using the
homebrew-provided version of SuiteSparse.

As suggested in the discussion of the issue, I added the
`CVXOPT_SUITESPARSE_INC_DIR` environment variable in the `.homebrew-
build-env` file.

This is an ugly workaround, and should probably be reported/discussed
also on the cvxopt repo, since the issue seems to be that they do
`#include "umfpack.h"` instead of a more proper `#include
"suitesparse/umfpack.h"`.
Indeed, for Linux system, their `setup.py` script then has to set the
include folder to be `whatever/include/suitesparse` instead of the
default system one.

Also, I don't have a way to check if this breaks other installations.

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URL: #37161
Reported by: Riccardo Zanotto
Reviewer(s): John H. Palmieri

.homebrew-build-env

@@ -45,3 +45,9 @@ for l in ; do
     fi
 done
 export ACLOCAL_PATH
+
+# Add cvxopt library path
+if [ -d "$HOMEBREW/include/suitesparse" ]; then
+    export CVXOPT_SUITESPARSE_INC_DIR="$HOMEBREW/include/suitesparse"
+    export CVXOPT_SUITESPARSE_LIB_DIR="$HOMEBREW/lib"
+fi