Pass -std=c99 flag to compiler

[concatime]

Hi there!
I'm quite new to Janet and JPM I must admit.
While executing jpm build (with a rather old GCC), I got:

[admin@localhost hermes]$ jpm build
compiling src/hermes.c to build/src___hermes.o...
src/hermes.c: In function ‘validate_pkg’:
src/hermes.c:75:9: error: ‘for’ loop initial declarations are only allowed in C99 mode
src/hermes.c:75:9: note: use option -std=c99 or -std=gnu99 to compile your code
src/hermes.c:110:5: error: ‘for’ loop initial declarations are only allowed in C99 mode
src/hermes.c:111:5: error: ‘for’ loop initial declarations are only allowed in C99 mode
src/hermes.c:112:5: error: ‘for’ loop initial declarations are only allowed in C99 mode
error: command exited with status 1
  in shell [/usr/local/bin/jpm] (tailcall) on line 143, column 5
  in do-rule [/usr/local/bin/jpm] on line 262, column 26
  in do-rule [/usr/local/bin/jpm] on line 258, column 44
  in do-rule [/usr/local/bin/jpm] (tailcall) on line 258, column 44

How do we pass -std=c99? (env CFLAGS=-std=c99 jpm build does not work)

Thank you!