Pass compiler and linker flags through

The OpenBSD port needs to control these flags, for example: anything
that executes writeable memory must be compiled with flags acknowledging
this, and Webkit is such a program.

This patch was written by Jeremy Evans:
https://marc.info/?l=openbsd-ports&m=147448331931201&w=2

src/webkit_server.pro

@@ -1,6 +1,9 @@
 TEMPLATE = app
 TARGET = webkit_server
 DESTDIR = .
+QMAKE_CXXFLAGS += $$(CXXFLAGS)
+QMAKE_CFLAGS += $$(CFLAGS)
+QMAKE_LFLAGS += $$(LDFLAGS)
 PROJECT_DIR = $$_PRO_FILE_PWD_
 BUILD_DIR = $${PROJECT_DIR}/build
 PRECOMPILED_DIR = $${BUILD_DIR}