math_opt: Fix XCode build

an XCode target can't have two source file with the same name since xcode flatten the directory path...

ortools/math_opt/CMakeLists.txt

@@ -29,6 +29,7 @@ set(NAME ${PROJECT_NAME}_math_opt)
 add_library(${NAME} OBJECT)
 target_sources(${NAME} PUBLIC
   $<TARGET_OBJECTS:${NAME}_core>
+  $<TARGET_OBJECTS:${NAME}_core_c_api>
   $<TARGET_OBJECTS:${NAME}_cpp>
   $<TARGET_OBJECTS:${NAME}_io>
   $<TARGET_OBJECTS:${NAME}_labs>

ortools/math_opt/core/CMakeLists.txt

@@ -11,8 +11,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+add_subdirectory(c_api)
+
 file(GLOB _SRCS "*.h" "*.cc")
-list(APPEND _SRCS "${CMAKE_CURRENT_SOURCE_DIR}/c_api/solver.cc")
+#list(APPEND _SRCS "${CMAKE_CURRENT_SOURCE_DIR}/c_api/solver.cc")
 list(FILTER _SRCS EXCLUDE REGEX "/[^/]*_test\\.cc$")
 
 set(NAME ${PROJECT_NAME}_math_opt_core)

ortools/math_opt/core/c_api/CMakeLists.txt

@@ -13,7 +13,6 @@
 
 set(NAME ${PROJECT_NAME}_math_opt_core_c_api)
 add_library(${NAME} OBJECT)
-
 target_sources(${NAME} PRIVATE solver.h solver.cc)
 set_target_properties(${NAME} PROPERTIES POSITION_INDEPENDENT_CODE ON)
 target_include_directories(${NAME} PUBLIC