Fix error when running CMake configure multiple times with CPR_BUILD_TESTS, bump CMake min version to 3.18

[Anohkka]

Attempting to configure libcpr multiple times with CPR_BUILD_TESTS currently fails with

CMake Error at CMakeLists.txt:394 (add_subdirectory):
  The binary directory

    [...]/build/_deps/mongoose-build

  is already used to build a source directory.  It cannot be used to build
  source directory

    [...]/build/_deps/mongoose-src

  Specify a unique binary directory name.

The issue is that when we configure the project and call FetchContent_MakeAvailable(mongoose) for the first time, it works fine because there's no CMakeLists.txt in ${mongoose_SOURCE_DIR}, but when attempting to configure the project again, we already copied over our custom CMakeLists.txt to ${mongoose_SOURCE_DIR}, so FetchContent_MakeAvailable runs add_subdirectory which duplicates the target created by our own add_subdirectory call.

Setting SOURCE_SUBDIR to a nonexistent directory prevents FetchContent_MakeAvailable from running add_subdirectory. SOURCE_SUBDIR was added in CMake version 3.18, which was released on July 15, 2020.