Unbreak build with GCC 4.2

FreeBSD on mips*, powerpc*, riscv64, sparc64 still uses GCC 4.2 (patched) as default compiler which doesn't support -std=gnu90 alias.

FAILED: test-build-std-gnuc90@exe/test_build-pedantic.c.o
gcc42 -Itest-build-std-gnuc90@exe -I. -I.. -I../src -I../include -I/usr/local/include -pipe -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wextra -std=gnu99 -g -Wno-unused-parameter -g -fvisibility=hidden -Wmissing-prototypes -Wstrict-prototypes -std=gnu90 -Werror  -MD -MQ 'test-build-std-gnuc90@exe/test_build-pedantic.c.o' -MF 'test-build-std-gnuc90@exe/test_build-pedantic.c.o.d' -o 'test-build-std-gnuc90@exe/test_build-pedantic.c.o' -c ../test/build-pedantic.c
cc1: error: unrecognized command line option "-std=gnu90"