Fix CI: add POSITION_INDEPENDENT_CODE for Linux shared library linking

The dtwc++ static library is linked into the Python .so extension.
On Linux, static libraries linked into shared objects must be compiled
with -fPIC. Set POSITION_INDEPENDENT_CODE ON for both dtwc++ and
mip-solvers targets.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ElektrikAkar

dtwc/CMakeLists.txt

@@ -2,6 +2,9 @@ cmake_minimum_required(VERSION 3.21)
 
 add_library(dtwc++ STATIC)
 
+# Required for linking into the Python shared library (.so) on Linux
+set_target_properties(dtwc++ PROPERTIES POSITION_INDEPENDENT_CODE ON)
+
 add_subdirectory(mip)
 
 target_sources(dtwc++

dtwc/mip/CMakeLists.txt

@@ -1,5 +1,6 @@
 cmake_minimum_required(VERSION 3.21)
 add_library(mip-solvers OBJECT)
+set_target_properties(mip-solvers PROPERTIES POSITION_INDEPENDENT_CODE ON)
 
 target_sources(mip-solvers
   PRIVATE