Build System not finding system versions of pybind installed by conda

[donaldwj]

Short description explaining the high-level reason for the new issue.

Current behavior

The ngen build system is installing its own copy of pybind when there is a system install used.

Expected behavior

The build system should use the system pybind install unless it is out of date.

Steps to replicate behavior (include URLs)

Screenshots

[program--]

As a note: this is expected behavior (currently) and doesn't have to do with conda. We intentionally use the vendored version of pybind11 (line 275):

ngen/CMakeLists.txt

Lines 271 to 276 in dd66466

	if(NGEN_WITH_PYTHON)
	add_compile_definitions(ACTIVATE_PYTHON=true)
	find_package(Python 3.6.8 REQUIRED COMPONENTS Interpreter Development NumPy)
	set(PYTHON_EXECUTABLE ${Python_EXECUTABLE}) # Case-sensitive difference
	add_subdirectory(extern/pybind11 pybind11)
	endif()

The easiest change/fix for this is probably replacing line 275 with a find_package(pybind11 CONFIG) call, and conditionally calling add_subdirectory if pybind11_FOUND is false.