cmake: force PIE executables + disable dynamic X11 on Linux wheels

This fixes the long-standing "invalid ELF header: RTLD_GLOBAL for idraw"
error that occurred with PyPI Linux wheels.

- Force CMAKE_POSITION_INDEPENDENT_CODE=ON for all executables.
  InterViews/idraw does dlopen(self, RTLD_GLOBAL|...), which modern
  glibc rejects for non-PIE (ET_EXEC) binaries.

- Disable IV_ENABLE_X11_DYNAMIC on Linux wheels only (SKBUILD && UNIX && NOT APPLE).
  The dynamic X11 loading path (ivx11_dynam.cpp) was the actual trigger
  of the (null) error. macOS wheels and local builds are unaffected.

No functional change for normal source builds or macOS.

Author: nrnhines

CMakeLists.txt

@@ -21,6 +21,22 @@ project(
   LANGUAGES C CXX
   HOMEPAGE_URL "https://nrn.readthedocs.io/")
 
+# =============================================================================
+# Fixes for Linux wheels
+# =============================================================================
+
+# Force PIE for all executables (idraw, nrniv, etc.)
+# This was the root cause of the original "invalid ELF header: RTLD_GLOBAL" error.
+cmake_policy(SET CMP0083 NEW)
+set(CMAKE_POSITION_INDEPENDENT_CODE ON CACHE BOOL "Force PIE for NEURON + InterViews" FORCE)
+
+# Disable fragile dynamic X11 loading *only* for Linux wheels.
+# This was triggering the (null) RTLD_GLOBAL error.
+# macOS wheels and normal/local Linux builds keep the original default (ON).
+if(SKBUILD AND UNIX AND NOT APPLE)
+  set(IV_ENABLE_X11_DYNAMIC OFF CACHE BOOL "Disable on Linux wheels" FORCE)
+endif()
+
 # =============================================================================
 # CMake common project settings
 # =============================================================================