Allow Rust to build core as a static lib (#3283)

Author: nyurik

CMakeLists.txt

@@ -1601,3 +1601,5 @@ endif()
 add_subdirectory(${PROJECT_SOURCE_DIR}/test)
 add_subdirectory(${PROJECT_SOURCE_DIR}/benchmark)
 add_subdirectory(${PROJECT_SOURCE_DIR}/render-test)
+
+include(cmake/mbgl-core-deps.cmake)

cmake/mbgl-core-deps.cmake

@@ -0,0 +1,33 @@
+# this file defines a target that will generate a mbgl-core-deps.txt which includes the linker
+# flags that would need to be passed to an executable or library that links with
+# the static mbgl-core library
+
+set(DUMMY_FILE "${CMAKE_BINARY_DIR}/dummy.cpp")
+add_custom_command(
+    OUTPUT ${DUMMY_FILE}
+    COMMAND ${CMAKE_COMMAND} -E echo "int main() {}" > ${DUMMY_FILE}
+    COMMENT "Generating dummy.cpp"
+    VERBATIM
+)
+
+# https://stackoverflow.com/questions/34165365/retrieve-all-link-flags-in-cmake
+set(CMAKE_ECHO_STANDARD_LIBRARIES ${CMAKE_CXX_STANDARD_LIBRARIES})
+set(CMAKE_ECHO_FLAGS ${CMAKE_CXX_FLAGS})
+set(CMAKE_ECHO_LINK_FLAGS ${CMAKE_CXX_LINK_FLAGS})
+set(CMAKE_ECHO_IMPLICIT_LINK_DIRECTORIES ${CMAKE_CXX_IMPLICIT_LINK_DIRECTORIES})
+set(
+    CMAKE_ECHO_LINK_EXECUTABLE
+    "<CMAKE_COMMAND> -E echo \"<FLAGS> <LINK_FLAGS> <LINK_LIBRARIES>\" > <TARGET>"
+)
+
+add_executable(mbgl-core-deps EXCLUDE_FROM_ALL "${DUMMY_FILE}")
+target_link_libraries(mbgl-core-deps mbgl-core)
+add_custom_target(generate_dummy DEPENDS ${DUMMY_FILE})
+add_dependencies(mbgl-core-deps generate_dummy)
+
+set_target_properties(
+    mbgl-core-deps
+        PROPERTIES
+            LINKER_LANGUAGE ECHO
+            SUFFIX          ".txt"
+)