Handle matrix-structs as an ExternalProject instead of a submodule

.gitmodules

-[submodule "libs/matrix-structs"]
-	path = libs/matrix-structs
-	url = https://github.com/mujx/matrix-structs

CMakeLists.txt

@@ -7,12 +7,6 @@ option(BUILD_LIB_EXAMPLES "Build examples" ON)
 
 set(CMAKE_INCLUDE_CURRENT_DIR ON)
 
-if (NOT EXISTS "${CMAKE_SOURCE_DIR}/libs/matrix-structs/.git")
-    message(WARNING "The git submodules are not available.")
-    message(STATUS "Running git submodule update --init --recursive ...")
-    execute_process(COMMAND git submodule update --init --recursive)
-endif()
-
 if(CMAKE_CXX_COMPILER_ID MATCHES "Clang" OR CMAKE_CXX_COMPILER_ID MATCHES "GNU")
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} \
         -std=c++1z \
@@ -67,11 +61,10 @@ include_directories(${Boost_INCLUDE_DIRS})
 #
 # matrix-structs
 #
-add_subdirectory(libs/matrix-structs)
-include_directories(${matrix_structs_SOURCE_DIR}/include)
-include_directories(${matrix_structs_SOURCE_DIR}/deps)
+include(${CMAKE_SOURCE_DIR}/cmake/MatrixStructs.cmake)
 
 include_directories(src)
+include_directories(${MATRIX_STRUCTS_INCLUDE_DIRS})
 set(SRC src/client.cpp src/utils.cpp)
 
 add_library(matrix_client ${SRC})

cmake/MatrixStructs.cmake

+include(ExternalProject)
+
+#
+# Build matrix-structs.
+#
+
+set(THIRD_PARTY_ROOT ${CMAKE_SOURCE_DIR}/.third-party)
+set(MATRIX_STRUCTS_ROOT ${THIRD_PARTY_ROOT}/matrix_structs)
+
+set(MATRIX_STRUCTS_INCLUDE_DIRS ${MATRIX_STRUCTS_ROOT}/deps)
+
+ExternalProject_Add(
+  MatrixStructs
+
+  GIT_REPOSITORY https://github.com/mujx/matrix-structs
+  GIT_TAG 83be1388e632a43f0570857cb79313c09fb3da0b
+
+  BUILD_IN_SOURCE 1
+  SOURCE_DIR ${MATRIX_STRUCTS_ROOT}
+  CONFIGURE_COMMAND ${CMAKE_COMMAND} -DCMAKE_BUILD_TYPE=Release ${MATRIX_STRUCTS_ROOT}
+  BUILD_COMMAND ${CMAKE_COMMAND} --build ${MATRIX_STRUCTS_ROOT}
+  INSTALL_COMMAND ""
+)
+
+include_directories(SYSTEM ${MATRIX_STRUCTS_ROOT}/deps)
+include_directories(SYSTEM ${MATRIX_STRUCTS_ROOT}/include)
+link_directories(${MATRIX_STRUCTS_ROOT})

matrix-structs @ 83be1388

-Subproject commit 83be1388e632a43f0570857cb79313c09fb3da0b