Cannot use c lib math.h



I am trying to call trig functions from a c file, but I keep getting:
mtest.c:(.text+0x74): undefined reference to `sin’
collect2: error: ld returned 1 exit status
My make file is:
all: hello-cpp-world mtest
g++ -std=c++11 < -o @
%: %.c
gcc < -o -lm @
None of the solutions I find online suggesting changes to the make file work.

What do I need to do?