build system: use cmake fetch content rather than external add

cmake/in/cereal.in

@@ -1,20 +0,0 @@
-project(cereal-download NONE)
-cmake_minimum_required(VERSION 3.2)
-
-include(ExternalProject)
-
-ExternalProject_Add(
-    cereal
-    GIT_REPOSITORY https://github.com/USCiLab/cereal.git
-    GIT_TAG v1.2.2
-    GIT_SHALLOW 1
-    DOWNLOAD_DIR ${CEREAL_DEPS_DIR}
-    TMP_DIR ${CEREAL_DEPS_DIR}/tmp
-    STAMP_DIR ${CEREAL_DEPS_DIR}/stamp
-    SOURCE_DIR ${CEREAL_DEPS_DIR}/src
-    BINARY_DIR ${CEREAL_DEPS_DIR}/build
-    CONFIGURE_COMMAND ""
-    BUILD_COMMAND ""
-    INSTALL_COMMAND ""
-    TEST_COMMAND ""
-)

cmake/in/cr.in

@@ -1,20 +0,0 @@
-project(cr-download NONE)
-cmake_minimum_required(VERSION 3.2)
-
-include(ExternalProject)
-
-ExternalProject_Add(
-    cr
-    GIT_REPOSITORY https://github.com/fungos/cr.git
-    GIT_TAG master
-    GIT_SHALLOW 1
-    DOWNLOAD_DIR ${CR_DEPS_DIR}
-    TMP_DIR ${CR_DEPS_DIR}/tmp
-    STAMP_DIR ${CR_DEPS_DIR}/stamp
-    SOURCE_DIR ${CR_DEPS_DIR}/src
-    BINARY_DIR ${CR_DEPS_DIR}/build
-    CONFIGURE_COMMAND ""
-    BUILD_COMMAND ""
-    INSTALL_COMMAND ""
-    TEST_COMMAND ""
-)

cmake/in/googletest.in

@@ -1,20 +0,0 @@
-project(googletest-download NONE)
-cmake_minimum_required(VERSION 3.2)
-
-include(ExternalProject)
-
-ExternalProject_Add(
-    googletest
-    GIT_REPOSITORY https://github.com/google/googletest.git
-    GIT_TAG master
-    GIT_SHALLOW 1
-    DOWNLOAD_DIR ${GOOGLETEST_DEPS_DIR}
-    TMP_DIR ${GOOGLETEST_DEPS_DIR}/tmp
-    STAMP_DIR ${GOOGLETEST_DEPS_DIR}/stamp
-    SOURCE_DIR ${GOOGLETEST_DEPS_DIR}/src
-    BINARY_DIR ${GOOGLETEST_DEPS_DIR}/build
-    CONFIGURE_COMMAND ""
-    BUILD_COMMAND ""
-    INSTALL_COMMAND ""
-    TEST_COMMAND ""
-)