meson: lower the minimum buildsystem requirements even more

Only a couple trivial features from meson ~0.50 were being used, and
none of them are really needed:

- configure_file() with the install kwarg has always defaulted to
  inferring its value from whether an install_dir was defined. This is
  fine, we don't need to set `install: true` in that case. The kwarg was
  only even added to meson 0.50 for consistency and to allow
  conditionally overriding the file to not install, even when
  install_dir is set. This project does not need that feature.

- path building could historically be done with the join_paths()
  function. Recent versions of meson (0.49) added cosmetic sugar in the
  form of string operator overloading to allow using the division
  operator on two strings. By removing this and using the backwards
  compatible form, we can support older versions of meson.

- sdl2 dependency lookup with hardcoded config-tool method is very
  opinionated about the correct way to look up sdl2, but meson can try
  multiple methods if you permit it, and there is no reason to think
  that config-tool is the only one that returns correct results.

By removing these features, the minimum can be dropped all the way down
to a version that is available on the oldest supported versions of
Ubuntu (18.04), Debian (oldoldstable / Stretch) and anywhere else of
consequence.

meson.build

@@ -2,7 +2,7 @@ project('lite-xl',
     ['c'],
     version : '2.0.3',
     license : 'MIT',
-    meson_version : '>= 0.50',
+    meson_version : '>= 0.42',
     default_options : ['c_std=gnu11']
 )
 
@@ -52,7 +52,7 @@ if not get_option('source-only')
     )
     pcre2_dep = dependency('libpcre2-8')
     freetype_dep = dependency('freetype2')
-    sdl_dep = dependency('sdl2', method: 'config-tool')
+    sdl_dep = dependency('sdl2')
     lite_deps = [lua_dep, sdl_dep, freetype_dep, pcre2_dep, libm, libdl, threads_dep]
 endif
 #===============================================================================
@@ -94,17 +94,16 @@ endif
 
 install_data('licenses/licenses.md', install_dir : lite_docdir)
 
-install_subdir('data' / 'core' , install_dir : lite_datadir, exclude_files : 'start.lua')
+install_subdir('data/core' , install_dir : lite_datadir, exclude_files : 'start.lua')
 foreach data_module : ['fonts', 'plugins', 'colors']
-    install_subdir('data' / data_module , install_dir : lite_datadir)
+    install_subdir(join_paths('data', data_module), install_dir : lite_datadir)
 endforeach
 
 configure_file(
     input : 'data/core/start.lua',
     output : 'start.lua',
     configuration : conf_data,
-    install : true,
+    install_dir : join_paths(lite_datadir, 'core'),
-    install_dir : lite_datadir / 'core',
 )
 
 if not get_option('source-only')