SDL2_image needs libm to build SVG

Just to let you know there is a link failure if you build SDL2_Image with support for SVG, it won’t link on Linux as it’s lacking sqrtf symbol.

/bin/sh ./libtool  --tag=CC   --mode=link gcc  -I/usr/include/libpng16  -g -O2 -D_REENTRANT -I/usr/include/SDL2   -o showimage showimage.o -lSDL2
libtool: link: gcc -I/usr/include/libpng16 -g -O2 -D_REENTRANT -I/usr/include/SDL2 -o showimage showimage.o  ./.libs/libSDL2_image.a -lSDL2
/home/laguest/opt/free-ada-6.4.0/lib/gcc/x86_64-pc-linux-gnu/6.4.0/../../../../x86_64-pc-linux-gnu/bin/ld: ./.libs/libSDL2_image.a(IMG_svg.o): undefined reference to symbol 'sqrtf@@GLIBC_2.2.5'
/lib64/ error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
make: *** [Makefile:550: showimage] Error 1

This fixes it for me:

LDFLAGS=-lm ../../SDL2_image-2.0.2/configure