Improve the implementation of unix-like directories usage

Add a Meson option "portable" to choose between "portable" and unix-like
directories. Add information about this option in the README.

To determine the user's directory use the variable USERPROFILE only on
Windows and use HOME otherwise.

Implement the "portable" option in the package build script.

build.sh

@@ -1,6 +1,10 @@
 #!/bin/bash
 
-cflags="-Wall -O3 -g -std=gnu11 -fno-strict-aliasing -Isrc -Ilib/font_renderer"
+if [[ $1 == -portable ]]; then
+  cflags="-DLITE_XL_DATA_USE_EXEDIR"
+fi
+
+cflags+="-Wall -O3 -g -std=gnu11 -fno-strict-aliasing -Isrc -Ilib/font_renderer"
 cflags+=" $(pkg-config --cflags lua5.2) $(sdl2-config --cflags)"
 lflags="-static-libgcc -static-libstdc++"
 for package in libagg freetype2 lua5.2; do