mpip: fix package to depends on libunwind when +libunwind (#24007)

Added a dependency for mpip@3.5: when the libunwind is set to true (which is the default)
and '~setjmp' is set to False (which is also the default) to avoid a configure
time error from not finding libunwind.

Co-authored-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>

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

@@ -59,9 +59,8 @@ class Mpip(AutotoolsPackage):
     depends_on('python@:2', when='@3.4.1', type='build')
     depends_on('mpi')
 
-    #  Ideally would use libunwind, but provide backtrace and
-    #    setjmp functionality, if needed
-    #  depends_on('unwind')
+    #  '+setjmp' adds '--disable-libunwind' to the confiure args
+    depends_on('unwind', when='@3.5: +libunwind ~setjmp')
 
     @when('@3.5:')
     def configure_args(self):