Fix building without shared library

Signed-off-by: Miodrag Milanovic <mmicko@gmail.com>

libtrellis/CMakeLists.txt

diff --git a/libtrellis/CMakeLists.txt b/libtrellis/CMakeLists.txt
index 7f00324..4f3edb9 100644
--- a/libtrellis/CMakeLists.txt
+++ b/libtrellis/CMakeLists.txt
@@ -145,4 +145,6 @@
 install(DIRECTORY ../misc DESTINATION share/trellis)
 install(DIRECTORY ../util/common DESTINATION share/trellis/util)
 install(DIRECTORY ../timing/util DESTINATION share/trellis/timing)
-install(TARGETS pytrellis DESTINATION share/trellis/libtrellis)
+if (BUILD_SHARED)
+   install(TARGETS pytrellis DESTINATION share/trellis/libtrellis)
+endif()