textproc/p5-Text-BibTeX: fix build with GCC-based architectures

btparse/tests/namebug.c:193: error: 'for' loop initial declaration used outside C99 mode on GCC architectures.

Set USE_CSTD=c99 to fix build.

PR: 237254
Approved by: perl (maintainer timeout), mat (mentor)
Differential Revision: https://reviews.freebsd.org/D20114