libssh2: fix linking on macOS (#16012)

* libssh2: fix linking on macOS

* Fix libgit2 linkage to libssh2

var/spack/repos/builtin/packages/libgit2/package.py

@@ -57,6 +57,7 @@ class Libgit2(CMakePackage):
     # Build Dependencies
     depends_on('cmake@2.8:', type='build', when="@:0.28")
     depends_on('cmake@3.5:', type='build', when="@0.99:")
+    depends_on('pkgconfig', type='build')
 
     # Runtime Dependencies
     depends_on('libssh2', when='+ssh')

var/spack/repos/builtin/packages/libssh2/package.py

@@ -28,3 +28,9 @@ def cmake_args(self):
         spec = self.spec
         return [
             '-DBUILD_SHARED_LIBS=%s' % ('YES' if '+shared' in spec else 'NO')]
+
+    @run_after('install')
+    def darwin_fix(self):
+        # The shared library is not installed correctly on Darwin; fix this
+        if self.spec.satisfies('platform=darwin'):
+            fix_darwin_install_name(self.prefix.lib)