It should be safe to default to C99 now

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

src/Makefile.am

 bin_PROGRAMS   = light
 light_SOURCES  = main.c light.c light.h helpers.c helpers.h
 light_CPPFLAGS = -I../include -D_XOPEN_SOURCE=500
-light_CFLAGS   = -W -Wall -Wextra -pedantic -std=c89
+light_CFLAGS   = -W -Wall -Wextra -pedantic -std=c99
 
 install-exec-hook:
 	chmod 4755 $(DESTDIR)$(bindir)/light