externals: Add CPR as a submodule.

.gitmodules

@@ -28,3 +28,6 @@
 [submodule "externals/enet"]
     path = externals/enet
     url = https://github.com/lsalzman/enet
+[submodule "cpr"]
+    path = externals/cpr
+    url = https://github.com/whoshuu/cpr.git

externals/CMakeLists.txt

@@ -4,6 +4,12 @@
 add_library(catch-single-include INTERFACE)
 target_include_directories(catch-single-include INTERFACE catch/single_include)
 
+# CPR
+option(BUILD_TESTING OFF)
+option(BUILD_CPR_TESTS OFF)
+add_subdirectory(cpr)
+target_include_directories(cpr INTERFACE ./cpr/include)
+
 # Crypto++
 add_subdirectory(cryptopp)