Correctly set include path in CFLAGS and CXXFLAGS

It appears that setting `INCLUDEPATH` in qmake break things.
Because qmake add the include path using `-isystem`.

We don't need to extract include path and define  from PKGCONFIG_CFLAGS.
Updating `CFLAGS` and `CXXFLAGS` is enough. No need for complex things.

Fix #90
Fix kiwix/kiwix-build#260

Author: mgautierfr

kiwix-desktop.pro

@@ -136,17 +136,6 @@ unix {
 
 PKGCONFIG_CFLAGS = $$system(pkg-config --cflags $$PKGCONFIG_OPTION kiwix)
 
-PKGCONFIG_INCLUDEPATH = $$find(PKGCONFIG_CFLAGS, ^-I.*)
-PKGCONFIG_INCLUDEPATH ~= s/^-I(.*)/\\1/g
-
-PKGCONFIG_DEFINES = $$find(PKGCONFIG_CFLAGS, ^-D.*)
-PKGCONFIG_DEFINES ~= s/^-D(.*)/\\1/g
-
-PKGCONFIG_CFLAGS ~= s/^-[ID].*//g
-
-INCLUDEPATH *= $$PKGCONFIG_INCLUDEPATH
-DEFINES *= $$PKGCONFIG_DEFINES
-
 QMAKE_CXXFLAGS += $$PKGCONFIG_CFLAGS
 QMAKE_CFLAGS += $$PKGCONFIG_CFLAGS