I recently upgraded to Ubuntu 18.04 and when I tried to run polybar again I got the issue:
polybar: /usr/lib/x86_64-linux-gnu/libcurl.so.4: version `CURL_OPENSSL_3' not found (required by polybar)

I tried to build it again and got:

Scanning dependencies of target xpp
[ 1%] Linking CXX static library libxpp.a
Traceback (most recent call last):
File "/home/jade/.config/polybar/polybar/lib/xpp/generators/cpp_client.py", line 3163, in
from xcbgen.state import Module
File "/usr/lib/python2.7/dist-packages/xcbgen/state.py", line 7, in
from xcbgen import matcher
File "/usr/lib/python2.7/dist-packages/xcbgen/matcher.py", line 12, in
from xcbgen.xtypes import *
File "/usr/lib/python2.7/dist-packages/xcbgen/xtypes.py", line 1221, in
class EventStruct(Union):
File "/usr/lib/python2.7/dist-packages/xcbgen/xtypes.py", line 1239, in EventStruct
out = main.output['eventstruct']
KeyError: 'eventstruct'
lib/xpp/CMakeFiles/xpp.dir/build.make:68: recipe for target 'lib/xpp/libxpp.a' failed
make[2]: *** [lib/xpp/libxpp.a] Error 1
CMakeFiles/Makefile2:402: recipe for target 'lib/xpp/CMakeFiles/xpp.dir/all' failed
make[1]: *** [lib/xpp/CMakeFiles/xpp.dir/all] Error 2
Makefile:129: recipe for target 'all' failed
make: *** [all] Error 2