if I compile :

#include <math.h>

double Summx2 = 2.0;
double StandardDeviation;

int main (int argc, char **argv)
	{
		StandardDeviation = sqrt(Summx2);

		return 0;
	}

with :
gcc `pkg-config --cflags gtk+-3.0` -o "test" "test.c" `pkg-config --libs gtk+-3.0`

I receive
/usr/local/bin/ld: /tmp//ccQtgX7c.o: undefined reference to symbol 'sqrt@@FBSD_1.0'
/usr/local/bin/ld: /lib/libm.so.5: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status

Its only the problem part I need gtk for display the GUI, the full programm ist to long.