libc++1 from home:hawkeye116477:waterfox:build-depends project
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Add repository and install manually
For openSUSE Leap 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:hawkeye116477:waterfox:build-depends/openSUSE_Leap_15.4/home:hawkeye116477:waterfox:build-depends.repo zypper refresh zypper install libc++1