add -lm to CFLAGS

src/Makefile.am

@@ -1,7 +1,7 @@
 bin_PROGRAMS   = light
 light_SOURCES  = main.c light.c light.h helpers.c helpers.h impl/sysfs.c impl/sysfs.h impl/util.h impl/util.c impl/razer.h impl/razer.c
 light_CPPFLAGS = -I../include -D_GNU_SOURCE
-light_CFLAGS   = -W -Wall -Wextra -std=gnu99 -Wno-type-limits -Wno-format-truncation -Wno-unused-parameter -fcommon
+light_CFLAGS   = -W -Wall -Wextra -std=gnu99 -Wno-type-limits -Wno-format-truncation -Wno-unused-parameter -fcommon -lm
 
 if CLASSIC
 install-exec-hook: