- enable PIC for erfa static library (required when linking into a shared library)

--- src/detail/erfa/CMakeLists.txt.orig	2026-03-20 00:58:15 UTC
+++ src/detail/erfa/CMakeLists.txt
@@ -254,6 +254,9 @@
 
 add_library(heyoka_erfa STATIC "${HEYOKA_ERFA_SRC_FILES}")
 
+# Required when linking into a shared library.
+set_property(TARGET heyoka_erfa PROPERTY POSITION_INDEPENDENT_CODE ON)
+
 # Compile in C99 mode.
 target_compile_features(heyoka_erfa PRIVATE c_std_99)
 # Enforce vanilla C.