vita-headers: Use CMake to find python3.

On Unix systems, `python` is typically not available, while on Windows
`python3` is typically not available.

CMakeLists.txt

@@ -3,7 +3,7 @@
 # Distributed under the MIT License (http://opensource.org/licenses/MIT)
 #
 
-cmake_minimum_required(VERSION 3.2.0)
+cmake_minimum_required(VERSION 3.12)
 
 project(vitasdk)
 
@@ -70,6 +70,8 @@ endif()
 find_package(Git REQUIRED)
 list(APPEND CMAKE_MODULE_PATH "${PROJECT_SOURCE_DIR}/cmake")
 
+find_package(Python3 REQUIRED COMPONENTS Interpreter)
+
 # Toolchain arch target
 set(target_arch arm-vita-eabi)
 # Build date
@@ -508,10 +510,11 @@ ExternalProject_add(vita-headers
     GIT_TAG ${HEADERS_TAG}
     ${GIT_SHALLOW_SUPPORT}
     CONFIGURE_COMMAND ""
-    BUILD_COMMAND VITA_LIBS_GEN=${vita_libs_gen_command}
+    BUILD_COMMAND ${CMAKE_COMMAND} -E env
+        VITA_LIBS_GEN=${vita_libs_gen_command}
         ARCH=${binutils_prefix}
         BYPASS_INSTALL=1
-        python <SOURCE_DIR>/build.py <BINARY_DIR>
+        ${Python3_EXECUTABLE} <SOURCE_DIR>/build.py <BINARY_DIR>
     # Copy the generated .a files to the install directory
     INSTALL_COMMAND ${CMAKE_COMMAND} -DGLOB_PATTERN=<BINARY_DIR>/*/*/*a
     -DINSTALL_DIR=${CMAKE_INSTALL_PREFIX}/${target_arch}/lib