Add jpegli-static-obj OBJECT build target

[zeriyoshi]

Description

Adds an intermediate object target jpegli-static-obj for building the jpegli-static library.

This makes it easier to extract object files in external projects, allowing the building of completely static binaries.
Combined with the changes in PR #115, this will enable the creation of libraries using jpegli for platforms that require strict static linking, such as iOS applications.

include(FetchContent)

FetchContent_Declare(
    jpegli
    SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/third_party/jpegli
)
FetchContent_MakeAvailable(jpegli)

add_library(${CMAKE_PROJECT_NAME} STATIC
    ${SOURCES}
    $<TARGET_OBJECTS:jpegli-static-obj>
    $<TARGET_OBJECTS:jpegli-libjpeg-obj>
    $<TARGET_OBJECTS:hwy>
)
target_include_directories(${CMAKE_PROJECT_NAME} PRIVATE
    $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
    ${jpegli_BINARY_DIR}/lib/include/jpegli
    ${INCLUDE_DIRS}
)

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