[cmake] Problem with dependencies for configure checks

From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2021-06-03 13:54:57

Hi,

I've been having this problem with CMake in Boost.Filesystem, where I 
need to have the Boost::library targets available before 
Boost::filesystem target is defined. This is needed in order to perform 
configuration checks that will affect how Boost::filesystem will be 
defined. For example, see the check_cxx_source_compiles calls:

https://github.com/boostorg/filesystem/blob/af6ac28b5785820b93a2af7dd6e9b801232fec19/CMakeLists.txt#L23-L50

For the check_cxx_source_compiles calls, I would have liked to add 
targets like Boost::config and Boost::winapi to CMAKE_REQUIRED_LIBRARIES 
so that the checks are compiled with include directories of those 
dependencies added in the command line. This doesn't work because these 
targets may not be defined at the point when the configuration checks 
are performed.

My current workaround, as you can see, is to add the Boost root to 
CMAKE_REQUIRED_INCLUDES, where the boost directory with the common 
headers tree is located. However, this will not work if we're going to 
make that directory optional or remove it. Also, you can see that I'm 
reusing checks implemented in Boost.Config, for which I also had to hard 
code its relative location from Boost.Filesystem. If we're going to 
allow different libraries located in separate directory trees, this will 
also break.

My question is, can this be fixed and how?

I was wondering if it is possible to make the target generation 
two-stage, where the first stage identifies library locations, including 
directories with their headers, and the second stage would define build 
targets. This way all library directories would be available to the 
second stage. Although I can already see the problem with second order 
dependencies with this approach...