It's a pleasure for me to help you
I'm dumb, the header was found, ( I think ).
The error come from the g++ linking line, this one : "g++ -lSDL -lSDL_gfx -lSDL_ttf WaveView.o -o WaveView".
lt failed to find the library in the linking process.
So it should be related to "*.so" file not found in the linking process...
Just to help me to understand why, copy the content of the SDL library in your build directory.
Grab the .so by seeking them on your filesystem.
# ls -l /usr/lib |grep -i sdl
# ls -l /usr/lib64 | grep -i sdl
find the SDL directory and copy it in your home directory.
with something like this
# cp -Rv /usr/lib64/SDL .
# cp -Rv /usr/lib/SDL .
Check the header are available in your working directory too, it is not a problem to copy them.
And check you find the "SDL_init" function there :
# grep SDL_Init SDL/*.h
Check the .so are available too
# ls -l SDL/*|grep -i so
The "-I." in "g++ -c -DLINUX -I. -LSDL/lib -g WaveView.cpp -o WaveView.o" tell g++ to add the header from your working directory.
The "-L./SDL" tell g++ linker to add the ".so" file in the "./SDL" directory in your linker resolution.
Then try this compilation line ( I add -lasound which is link to alsa lib, -lpthread wich is link to pthread library, -LSDL which is add the SDL directory from the SDL lib directory in your build directory ) :
# g++ -I./SDL -L./SDL -lasound -lpthread -lSDL -lSDL_gfx -lSDL_ttf WaveView.o -o WaveView
If this fix the issue, it's great, I will try to resolve this in the git tonight.