Use pkgconfig to build against the system libusb and fftw

Desktop_Interface/Labrador.pro

@@ -127,14 +127,11 @@ win32{
 
 unix:!android:!macx{
     INCLUDEPATH += $$PWD/build_linux
-    LIBS += -lfftw3
+    CONFIG += link_pkgconfig
+    PKGCONFIG += libusb-1.0  ##make sure you have the libusb-1.0-0-dev package!
+    PKGCONFIG += fftw3
     contains(QT_ARCH, arm) {
             message("Building for Raspberry Pi")
-            #libusb include
-            unix:!android:!macx:LIBS += -lusb-1.0  ##make sure you have the libusb-1.0-0-dev package!
-            unix:!android:!macx:INCLUDEPATH += build_linux/libusb
-            unix:!android:!macx:DEPENDPATH += build_linux/libusb
-
             #libdfuprog include
             unix:!android:!macx:LIBS += -L$$PWD/build_linux/libdfuprog/lib/arm -ldfuprog-0.9
             unix:!android:!macx:INCLUDEPATH += $$PWD/build_linux/libdfuprog/include
@@ -149,10 +146,6 @@ unix:!android:!macx{
     } else {
         contains(QT_ARCH, i386) {
             message("Building for Linux (x86)")
-            unix:!android:!macx:LIBS += -lusb-1.0  ##make sure you have the libusb-1.0-0-dev package!
-            unix:!android:!macx:INCLUDEPATH += build_linux/libusb
-            unix:!android:!macx:DEPENDPATH += build_linux/libusb
-
             #libdfuprog include
             unix:!android:!macx:LIBS += -L$$PWD/build_linux/libdfuprog/lib/x86 -ldfuprog-0.9
             unix:!android:!macx:INCLUDEPATH += $$PWD/build_linux/libdfuprog/include
@@ -162,11 +155,6 @@ unix:!android:!macx{
 
         } else {
             message("Building for Linux (x64)")
-            #libusb include
-            unix:!android:!macx:LIBS += -Lbuild_linux/libusb -lusb-1.0  ##I suspect the -L here does nothing!
-            unix:!android:!macx:INCLUDEPATH += build_linux/libusb
-            unix:!android:!macx:DEPENDPATH += build_linux/libusb
-
             #libdfuprog include
             unix:!android:!macx:LIBS += -L$$PWD/build_linux/libdfuprog/lib/x64 -ldfuprog-0.9
             unix:!android:!macx:INCLUDEPATH += $$PWD/build_linux/libdfuprog/include