flexiblas: explicitly set SYSCONFDIR (#41748)

As of CMake 3.4, [GNUInstallDirs](https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html) treats `SYSCONFDIR` differently for a prefix that starts with `/opt`, then "the SYSCONFDIR value etc becomes /etc/opt/...." In the case of flexiblas, that results in failing attempts to write files to a system directory.

Since [flexiblas version 1](0f2d2c7659 (diff-1e7de1ae2d059d21e1dd75d5812d5a34b0222cef273b7c3a2af62eb747f9d20aR16)), we can override SYSCONFDIR with our own defines.

var/spack/repos/builtin/packages/flexiblas/package.py

@@ -22,3 +22,6 @@ class Flexiblas(CMakePackage):
     # virtual dependency
     provides("blas")
     provides("lapack")
+
+    def cmake_args(self):
+        return [self.define("SYSCONFDIR", self.prefix.etc)]