Fixed bug in Makefile

[buma]

-lm was missing

Without this I got an error in GCC 5.3.0:

gcc -Wall -Wextra -ansi -pedantic -std=c99 -O2 `gdal-config --cflags`  `gdal-config --libs`   gdaldem_web.c   -o gdaldem_web
/usr/bin/ld: /tmp/ccAJTcWc.o: undefined reference to symbol 'floorf@@GLIBC_2.2.5'
/usr/lib/libm.so.6: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
<builtin>: recipe for target 'gdaldem_web' failed
make: *** [gdaldem_web] Error