[BUILD] Download pybind11 in setup.py (#703)

Based on the discussion in #700, this PR enables downloading pybind11 in
`setup.py` without `git submodule` instead of copy-pasting pybind11
code. The downloaded pybind11 will be in `~/.triton/pybind` (like
`llvm`).

CMakeLists.txt

@@ -26,6 +26,9 @@ endif()
 # Compiler flags
 include_directories(${CMAKE_CURRENT_SOURCE_DIR}/include)
 
+# Third-party
+include_directories(${PYBIND11_INCLUDE_DIR})
+
 if(WIN32)
     SET(BUILD_SHARED_LIBS OFF)
     include_directories(${CMAKE_CURRENT_SOURCE_DIR}/deps/dlfcn-win32/src)
@@ -60,7 +63,7 @@ if("${LLVM_LIBRARY_DIR}" STREQUAL "")
 # sometimes we don't want to use llvm-config, since it may have been downloaded for some specific linux distros
 else()
     set(LLVM_LDFLAGS "-L${LLVM_LIBRARY_DIR}")
-    set(LLVM_LIBRARIES 
+    set(LLVM_LIBRARIES
 libLLVMNVPTXCodeGen.a
 libLLVMNVPTXDesc.a
 libLLVMNVPTXInfo.a