data: Generate .env files at build time

These files has been generated by the installer scripts for historical reasons.
Now that we have full-blown local builds and the upcoming MS Store version,
this pervasive pratice can't continue since it creates disparity between builds.

data/environ/default.env.in

@@ -1,2 +1,4 @@
 # Example entry in files like these
 # FOOPATH=/path/to/foo/stuff
+
+@BIN_PATH@

data/environ/meson.build

@@ -1,5 +1,30 @@
-install_data(
-  'default.env',
-  'python.env',
+environ_config = configuration_data()
+
+if platform_windows
+  environ_config.set('BIN_PATH', 'PATH=${gimp_installation_dir}\\bin')
+else
+  environ_config.set('BIN_PATH', '')
+endif
+
+configure_file(
+  input : 'default.env.in',
+  output: 'default.env',
+  configuration: environ_config,
+  install: true,
   install_dir: gimpplugindir / 'environ',
 )
+
+
+if not meson.is_cross_build() and python.found()
+  install_data(
+    'python.env',
+    install_dir: gimpplugindir / 'environ',
+  )
+endif
+
+if platform_windows and not meson.is_cross_build() and python.found()
+  install_data(
+    'pygimp.env',
+    install_dir: gimpplugindir / 'environ',
+  )
+endif

data/environ/pygimp.env

@@ -0,0 +1 @@
+__COMPAT_LAYER=HIGHDPIAWARE