Found while importing libomp into the FreeBSD base system (https://reviews.freebsd.org/D11507). Use of register produced warnings like:
register.cc:5:2: warning: 'register' storage class specifier is deprecated and incompatible with C++1z [-Wdeprecated-register] register int i; ^~~~~~~~~
The proposed FreeBSD import currently uses a hack (adding -Dregister= to CFLAGS). Here, just remove the deprecated keyword.