Clang build problems

[Allen Winter]

Can anyone else reproduce this?
I'm testing on Fedora 33 with clang version 11.0.0
no problems encountered with gcc10.2.1. 

== In the 3.0 branch ==
mkdir build; cd build
export CC=clang; export CXX=clang++
cmake .. && make && make test

The following tests FAILED:
          1 - regression (SEGFAULT)
         36 - recur (SEGFAULT)
         37 - icaltm_test (SEGFAULT)
         41 - timezones (SEGFAULT)
         42 - builtin_timezones (SEGFAULT)  (see backtrace attached. component is NULL?)

== In the master branch ==
mkdir build; cd build
export CC=clang; export CXX=clang++
cmake .. && make && make test

100% tests passed, 0 tests failed out of 43

-------------- next part --------------
(gdb)
#0  0x00007ffff772bafe in __strcmp_avx2 () from /lib64/libc.so.6
#1  0x00007ffff77dc7e5 in icalcomponent_isa_component (component=0xc00000000)
    at /home/allen/projects/libical/libical/src/libical/icalcomponent.c:321
#2  0x00007ffff77dbcfa in icalcomponent_add_children (impl=0x7ffff0003e70,
    args=0x7ffff5758eb0)
    at /home/allen/projects/libical/libical/src/libical/icalcomponent.c:76
#3  0x00007ffff77dbc64 in icalcomponent_vanew (kind=ICAL_XSTANDARD_COMPONENT)
    at /home/allen/projects/libical/libical/src/libical/icalcomponent.c:129
#4  0x00007ffff77f30b4 in icaltzutil_fetch_timezone (
    location=0x42b5b0 "Europe/London")
    at /home/allen/projects/libical/libical/src/libical/icaltz-util.c:864
#5  0x00007ffff77f5eaa in icaltimezone_load_builtin_timezone (zone=0x41ae18)
    at /home/allen/projects/libical/libical/src/libical/icaltimezone.c:1913
#6  0x00007ffff77f604d in icaltimezone_get_component (zone=0x41ae18)
    at /home/allen/projects/libical/libical/src/libical/icaltimezone.c:1243