Some build-checks ignored with cmake

[jasom]

Issue description

I first noticed this when running zeromq inside a chroot on nixos; it was trying to open /dev/urandom and failing. Building manually, I noticed that cmake checks for getrandom() in zmq_check_getrandom() but then never sets -DZMQ_HAVE_GETRANDOM at buildtime

Environment

• libzmq version (commit hash if unreleased): 4.3.2
• OS: Linux/NixOS

Minimal test code / Steps to reproduce the issue

1. Start with source on any system with getrandom()
2. mkdir build
3. cd build
4. cmake ..
   • Ensure the following in output:

     -- Performing Test ZMQ_HAVE_GETRANDOM
     -- Performing Test ZMQ_HAVE_GETRANDOM - Success
     

5. make VERBOSE=1 src/tweetnacl.o
6. grep getrandom CMakeFiles/objects.dir/src/tweetnacl.c.o
7. Build the whole system, strace any program that initializes zeromq

What's the actual result? (include assertion message & call stack if applicable)

In step #5 above, -DZMQ_HAVE_GETRANDOM does not appear on the command line
In step #6 above tweenacl,c.o has no match to the string "getrandom"
In step #7 above, observe that "/dev/urandom" is being opened, indicating getrandom is not in use

What's the expected result?

• We should see -DZMQ_HAVE_GETRANDOM on the build command-line
• grep should find a "getrandom" string in the tweentnacl binary file
• initializing zeromq should not open /dev/urandom

Patch that fixes the problem

The following patch fixes the issue for getrandom(). I don't know if it's the idiomatic way to do it with cmake, and I suspect that most of the other checks will need updating as well (e.g. ZMQ_HAVE_NOEXCEPT seems to similarly have no effect), so I didn't make a PR:

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8eb3411f..455369f5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -980,6 +980,10 @@ if(ZMQ_HAVE_TIPC)
   list(APPEND cxx-sources tipc_address.cpp tipc_connecter.cpp tipc_listener.cpp)
 endif()
 
+if(ZMQ_HAVE_GETRANDOM)
+  add_definitions(-DZMQ_HAVE_GETRANDOM)
+endif()
+
 #-----------------------------------------------------------------------------
 # source generators

[stale]

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