Possible issue with BUILD_INTERFACE

Code
2

$<BUILD_INTERFACE:...> expects a list as argument. For that purpose, use a; as separator of the arguments or, better, use genex $<SEMICOLON>. So the correct expression is:

target_include_directories(testlib
    PUBLIC
        $<BUILD_INTERFACE:${CMAKE_SOURCE_DIR}/include1$<SEMICOLON>${CMAKE_SOURCE_DIR}/include2>
        $<INSTALL_INTERFACE:include2>
    PRIVATE
        include
)