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Issue description

I encountered a zyre REGRESSION error when I was trying to update vcpkg’s zeromq to lastest: https://github.com/microsoft/vcpkg/pull/8119 In https://github.com/zeromq/libzmq/pull/3579, external/sha1/sha1.h and external/sha1/sha1.c are added. However, czmq also has its own copy of these two files: https://github.com/zeromq/czmq/tree/master/src/foreign/sha1

If both libzmq and czmq are built as static libraries, then the compiler will be unhappy:

/ci/myagent/_work/1/s/installed/x64-linux/debug/lib/libzmq.a(sha1.c.o):/ci/myagent/_work/1/s/buildtrees/zeromq/src/407c1e69ad-6a0d86f40e/external/sha1/sha1.c:225: first defined here
/ci/myagent/_work/1/s/installed/x64-linux/debug/lib/libczmq.a(zdigest.c.o): In function `sha1_pad':
/ci/myagent/_work/1/s/buildtrees/czmq/src/ff5f9b6893-f1fd59e8c7/src/foreign/sha1/sha1.inc_c:235: multiple definition of `sha1_pad'
/ci/myagent/_work/1/s/installed/x64-linux/debug/lib/libzmq.a(sha1.c.o):/ci/myagent/_work/1/s/buildtrees/zeromq/src/407c1e69ad-6a0d86f40e/external/sha1/sha1.c:236: first defined here
/ci/myagent/_work/1/s/installed/x64-linux/debug/lib/libczmq.a(zdigest.c.o): In function `sha1_loop':
/ci/myagent/_work/1/s/buildtrees/czmq/src/ff5f9b6893-f1fd59e8c7/src/foreign/sha1/sha1.inc_c:278: multiple definition of `sha1_loop'
/ci/myagent/_work/1/s/installed/x64-linux/debug/lib/libzmq.a(sha1.c.o):/ci/myagent/_work/1/s/buildtrees/zeromq/src/407c1e69ad-6a0d86f40e/external/sha1/sha1.c:279: first defined here
/ci/myagent/_work/1/s/installed/x64-linux/debug/lib/libczmq.a(zdigest.c.o): In function `sha1_result':
/ci/myagent/_work/1/s/buildtrees/czmq/src/ff5f9b6893-f1fd59e8c7/src/foreign/sha1/sha1.inc_c:306: multiple definition of `sha1_result'
/ci/myagent/_work/1/s/installed/x64-linux/debug/lib/libzmq.a(sha1.c.o):/ci/myagent/_work/1/s/buildtrees/zeromq/src/407c1e69ad-6a0d86f40e/external/sha1/sha1.c:307: first defined here
collect2: error: ld returned 1 exit status
[19/25] : && /usr/bin/c++  -fPIC -g  -rdynamic CMakeFiles/ztester_gossip.dir/src/ztester_gossip.c.o  -o ztester_gossip  libzyre.a /ci/myagent/_work/1/s/installed/x64-linux/debug/lib/libzmq.a /ci/myagent/_work/1/s/installed/x64-linux/debug/lib/libczmq.a /ci/myagent/_work/1/s/installed/x64-linux/debug/lib/libzmq.a -lrt -lpthread && :
FAILED: ztester_gossip 

Environment

• libzmq version (e756743a12468fabe0141f979aa2da407c1e69ad):
• OS: Ubuntu 19.04

Minimal test code / Steps to reproduce the issue

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

What’s the expected result?