Add conditional build option for stb-image implementation

[magicse]

Added option to conditionally include stb-image implementation in build. To avoide multiple definition of `stbi_load' and e.t.c....

Overview

Additional information

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[GencoreOperative]

I've provided a comment to consider. Thanks for looking into this.

[GencoreOperative]

An alternative approach is to lean on the CMake provided WIN32 flag which felt like a possible fit as well.

Suggested change

	option(LLAMA_AGENT_NEEDS_STB "Build stb_image implementation locally" OFF)
	if(LLAMA_AGENT_NEEDS_STB)
	list(APPEND AGENT_SOURCES stb-image-impl.cpp)
	endif()
	# stb-image-impl.cpp compiles STB_IMAGE_IMPLEMENTATION into the executable. On
	# non-Windows static builds these stbi_* symbols are already provided by libmtmd.a
	# (pulled in transitively via server-context), so compiling them here causes
	# duplicate-definition link errors. Only build it on Windows, where llama-agent
	# links against mtmd's shared library and needs its own copy.
	if(WIN32)
	target_sources(${TARGET} PRIVATE stb-image-impl.cpp)
	endif()