Build ErrorExtern into the extension module

ErrorExtern.cpp defines ccsp_set_error but was only listed in the
test_c_api target, so the six other C API translation units in cspimpl
referenced an undefined symbol. ELF and Mach-O permit undefined symbols
in a shared object and resolve them lazily, so the Linux and macOS
wheels linked and only Windows failed, with LNK1120 on _cspimpl.pyd.
The module would have raised ImportError on first use everywhere.

Also define CSPIMPL_EXPORTS for test_c_api. That target compiles the C
API sources itself, so without it CSP_C_API_EXPORT expands to dllimport
for symbols the target defines locally, which MSVC reports as LNK4217
and LNK4286.

Signed-off-by: Tim Paine <3105306+timkpaine@users.noreply.github.com>

Author: timkpaine

cpp/csp/python/CMakeLists.txt

@@ -88,6 +88,7 @@ add_library(cspimpl SHARED
         # and land directly in the extension module rather than needing the archive force-loaded.
         ../engine/AdapterManagerExtern.cpp
         ../engine/DictionaryExtern.cpp
+        ../engine/ErrorExtern.cpp
         ../engine/OutputAdapterExtern.cpp
         ../engine/PushInputAdapterExtern.cpp
         ../engine/StringExtern.cpp

cpp/tests/engine/CMakeLists.txt

@@ -26,6 +26,9 @@ add_executable(test_c_api
         ../../csp/engine/StringExtern.cpp
         ../../csp/engine/TimeExtern.cpp
         ../../csp/engine/ValueExtern.cpp)
+# These sources define the C API symbols, so the export macro must resolve to dllexport rather
+# than dllimport; without this MSVC emits LNK4217/LNK4286 for every locally defined symbol.
+target_compile_definitions(test_c_api PRIVATE CSPIMPL_EXPORTS)
 target_link_libraries(test_c_api GTest::gtest GTest::gtest_main)
 
 install(TARGETS test_tick_buffer test_time_series test_partial_switch_csp_type test_dictionary test_window_buffer test_c_api