mail/jmba: don't auto-enable gettext

11.0 aarch64 used ld(1) from devel/binutils which implicitly passed
-L/usr/local/lib thus gettext ended up being detected, breaking build
due to underspecified USES=gettext-tools. The same issue would occur
for USES=localbase or USE_GCC=yes, so just force off external gettext.

Approved by: portmgr blanket