Make building own libsamplerate and pybind11 optional

And make it not default.
Fixes: GH-24

Author: uis246

CMakeLists.txt

@@ -10,13 +10,17 @@ set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
 cmake_policy(SET CMP0094 NEW)
 
-# adds the external dependencies
-add_subdirectory(external)
+option(USE_SYSTEM_LIBRARIES "Use system libsamplerate and pybind instead of building separate versions" ON)
+if(USE_SYSTEM_LIBRARIES)
+	find_package(pybind11 REQUIRED)
+else()
+	# adds the external dependencies
+	add_subdirectory(external)
+	target_include_directories(python-samplerate PRIVATE ./external/libsamplerate/include)
+endif()
 
 pybind11_add_module(python-samplerate src/samplerate.cpp)
 
-target_include_directories(python-samplerate PRIVATE ./external/libsamplerate/include)
-
 if(MSVC)
     target_compile_options(python-samplerate PRIVATE /EHsc /MP /bigobj)
     set(CMAKE_EXE_LINKER_FLAGS /MANIFEST:NO)