CMake combine multiple precompiled lib files to single target - c++

Question

We are using cmake and started using conan as a package manager.
Before conan however prebuild 3rd Party libs like GTest where added via
find_package(GTest REQUIRED). (By adding custom find Scripts for each library find_package() was able to locate our prebuild third parties)
In the new conan way it's pretty easy to add 3rd parties to a library
add_library(myLib sources)
target_link_libraries(myLib PRIVATE GTEST::GTEST)
To achieve backwards compatibility i need to create a target with the name GTest::GTest from the find_package().
Now the question is how can i create a new target from the find_package() result?
I did try to create a new dummy target
find_package(GTest REQUIRED)
add_library("GTest" "foo.cpp")
target_link_libraries("GTest" PUBLIC "${GTest_LIBRARIES}")
add_library("GTEST::GTEST" ALIAS GTest)
where foo.cpp is just an empty file.
when linking against the target GTEST::GTEST
this results in a linker error GTEST_1_0d.lib was not found
How would you create a cmake target from prebuild binaries?
Could you help me out here?
Edit: Sorry for beeing unclear.
I do not want to touch legacy systems. Therefore Conan is not available on legacy systems.
The question is: How can I create targets from precompiled libraries that look exactly like Conan targets

Answer 1

To achieve backwards compatibility i need to create a target with the name GTest::GTest from the find_package(). Now the question is how can i create a new target from the find_package() result?
As you say, the result of your call to find_package(GTest) is that GTEST::GTEST is created. So just write
add_library(GTest::GTest ALIAS GTEST::GTEST)
If you have code that needs to use the other name, no?
On the other hand:
find_package(GTest REQUIRED)
If the result of your call to find_package(GTest) is the ancient built-in CMake module, then something like this might work:
find_package(GTest REQUIRED)
# Not using Conan
if (NOT TARGET GTEST::GTEST)
if (TARGET GTest::gtest)
# CMake 3.20+ module in use
add_library(GTEST::GTEST ALIAS GTest::gtest)
elseif (TARGET GTest::GTest)
# CMake 3.5+ module in use
add_library(GTEST::GTEST ALIAS GTest::GTest)
else ()
add_library(GTEST::GTEST IMPORTED INTERFACE)
target_include_directories(GTEST::GTEST INTERFACE "${GTEST_INCLUDE_DIRS}")
target_link_libraries(GTEST::GTEST INTERFACE "${GTEST_LIBRARIES}")
endif ()
endif ()
You can decide whether to add GTest::gtest_main, GTest::Main, or use ${GTEST_BOTH_LIBRARIES} in each of the three respective branches.