project file allow installation in another dir than "/usr"

E.g. this way I can build and install in "~/.local", or "/usr/local".

app/app.pro

@@ -19,20 +19,26 @@ RESOURCES += qml/resources.qrc
 ##              INSTALLS
 #########################################
 
-target.path += /usr/bin/
+PREFIX = $$(PREFIX) # Pass the make install PREFIX via environment variable. E.g. "PREFIX=/path/to/my/dir qmake".
+isEmpty(PREFIX) {
+    message(No prefix given. Using /usr.)
+    PREFIX=/usr
+}
+
+target.path = $$PREFIX/bin
 
 INSTALLS += target
 
 # Install icons
 unix {
     icon32.files = icons/32x32/cool-retro-term.png
-    icon32.path = /usr/share/icons/hicolor/32x32/apps
+    icon32.path = $$PREFIX/share/icons/hicolor/32x32/apps
     icon64.files = icons/64x64/cool-retro-term.png
-    icon64.path = /usr/share/icons/hicolor/64x64/apps
+    icon64.path = $$PREFIX/share/icons/hicolor/64x64/apps
     icon128.files = icons/128x128/cool-retro-term.png
-    icon128.path = /usr/share/icons/hicolor/128x128/apps
+    icon128.path = $$PREFIX/share/icons/hicolor/128x128/apps
     icon256.files = icons/256x256/cool-retro-term.png
-    icon256.path = /usr/share/icons/hicolor/256x256/apps
+    icon256.path = $$PREFIX/share/icons/hicolor/256x256/apps
 
     INSTALLS += icon32 icon64 icon128 icon256
 }

cool-retro-term.pro

@@ -5,7 +5,17 @@ CONFIG += ordered
 SUBDIRS += qmltermwidget
 SUBDIRS += app
 
+#########################################
+##              INSTALLS
+#########################################
+
+PREFIX = $$(PREFIX) # Pass the make install PREFIX via environment variable. E.g. "PREFIX=/path/to/my/dir qmake".
+isEmpty(PREFIX) {
+    message(No prefix given. Using /usr.)
+    PREFIX=/usr
+}
+
 desktop.files += cool-retro-term.desktop
-desktop.path += /usr/share/applications
+desktop.path += $$PREFIX/share/applications
 
 INSTALLS += desktop