Backports some changes sent upstream to better support plain cmake builds

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 1cc283d..5795a97 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -15,7 +15,10 @@ if (UNIX AND NOT APPLE)
     set(LINUX TRUE)
 endif()
 
-if (EXISTS "${CMAKE_SOURCE_DIR}/.version")
+if (DEFINED BUILD_VERSION)
+    message("Using build version from command-line")
+    set(VERSION_STRING "${BUILD_VERSION}")
+elseif (EXISTS "${CMAKE_SOURCE_DIR}/.version")
     message("Getting build version from .version file")
     file (STRINGS "${CMAKE_SOURCE_DIR}/.version" BUILD_VERSION)
     string(REGEX REPLACE "\n$" "" BUILD_VERSION "${BUILD_VERSION}")
@@ -106,7 +143,13 @@ else()
 	endif()
 
 	# PFFFT
-	find_package(PFFFT REQUIRED)
+	option(USE_SYSTEM_PFFFT "Use a system-provided version of pffft" OFF)
+	if(USE_SYSTEM_PFFFT)
+		message(STATUS "Using system pffft")
+		find_package(PFFFT REQUIRED)
+	else()
+		add_subdirectory(external)
+	endif()
 
 endif()
 
@@ -124,7 +167,7 @@ elseif( MANUAL_PYTHON_PATH )
 	set ( Python3_ROOT_DIR "${MANUAL_PYTHON_PATH}" )
 endif()
 
-find_package (Python3 3.9 EXACT COMPONENTS Interpreter Development REQUIRED)
+find_package (Python3 3.9 COMPONENTS Interpreter Development REQUIRED)
 message("Python3 Libs: ${Python3_LIBRARIES}")
 
 #
diff --git a/cmake/PackExternalDeps.cmake b/cmake/PackExternalDeps.cmake
index 4be07c7..330be03 100644
--- a/cmake/PackExternalDeps.cmake
+++ b/cmake/PackExternalDeps.cmake
@@ -145,7 +145,7 @@ endfunction(pack_python)
 function(pack_pffft)
 	install(FILES ${PFFFT_LIBRARIES}
 			DESTINATION "${CMAKE_INSTALL_LIBDIR}")
-endfunction(pack_pcap)
+endfunction(pack_pffft)
 
 function(pack_pcap)
     if (LINUX)
diff --git a/external/CMakeLists.txt b/external/CMakeLists.txt
new file mode 100644
index 0000000..ea1c9a3
--- /dev/null
+++ b/external/CMakeLists.txt
@@ -0,0 +1,17 @@
+
+# can't assume the target system has the required SIMD instructions
+option(PFFFT_ENABLE_SIMD "Enable SIMD support for pffft" OFF)
+
+message(STATUS "Using bundled pffft")
+add_library(pffft STATIC pffft/pffft.c)
+if(PFFFT_ENABLE_SIMD)
+    message(STATUS "Enabling SIMD support for pffft")
+else()
+    message(STATUS "Disabling SIMD support for pffft")
+    target_compile_definitions(pffft PUBLIC PFFFT_SIMD_DISABLE)
+endif()
+target_compile_options(pffft PUBLIC "${PFFFT_CFLAGS}")
+target_link_libraries(pffft m)
+target_include_directories(pffft INTERFACE pffft)
+
+add_library(PFFFT::PFFFT ALIAS pffft)