FetchContent for dependencies (#15)

* Use FetchContent for dependencies

* Remove submodules

* 0.10.0

.gitmodules

@@ -1,6 +0,0 @@
-[submodule "external/litexx"]
-	path = external/litexx
-	url = https://github.com/seanmiddleditch/litexx
-[submodule "external/doctest"]
-	path = external/doctest
-	url = https://github.com/onqtam/doctest

CMakeLists.txt

@@ -1,7 +1,7 @@
 cmake_minimum_required(VERSION 3.8)
 
 project(formatxx
-    VERSION 0.9.0
+    VERSION 0.10.0
     LANGUAGES CXX
 )
 

external/CMakeLists.txt

@@ -1,9 +1,25 @@
-if(NOT TARGET litexx)
-    add_subdirectory(litexx EXCLUDE_FROM_ALL)
-endif()
+include(FetchContent)
+FetchContent_Declare(doctest
+    GIT_REPOSITORY https://github.com/onqtam/doctest.git
+    GIT_TAG 2.3.1
+    GIT_SHALLOW ON
+    GIT_SUBMODULES ""
+)
+FetchContent_Declare(litexx
+    GIT_REPOSITORY https://github.com/seanmiddleditch/litexx.git
+    GIT_TAG 0.1.0
+    GIT_SHALLOW ON
+    GIT_SUBMODULES ""
+)
 
 if(FORMATXX_BUILD_TESTS AND NOT TARGET doctest)
+    FetchContent_Populate(doctest)
     set(DOCTEST_WITH_TESTS OFF CACHE BOOL "enable doctest tests")
     set(DOCTEST_WITH_MAIN_IN_STATIC_LIB OFF CACHE BOOL "enable doctest static library")
-    add_subdirectory(doctest EXCLUDE_FROM_ALL)
+    add_subdirectory(${doctest_SOURCE_DIR} ${doctest_BINARY_DIR} EXCLUDE_FROM_ALL)
+endif()
+
+if(NOT TARGET litexx)
+    FetchContent_Populate(litexx)
+    add_subdirectory(${litexx_SOURCE_DIR} ${litexx_BINARY_DIR} EXCLUDE_FROM_ALL)
 endif()