rpm: use build flags provided by the distribution

Among other things, this enable various hardening options.

QubesOS/qubes-issues#2259

(cherry picked from commit 7667b0dc16)

qrexec/Makefile

@@ -1,7 +1,7 @@
-CC=gcc
-CFLAGS+=-I. -g -O2 -Wall -Wextra -Werror -pie -fPIC `pkg-config --cflags vchan-$(BACKEND_VMM)`
-LDFLAGS=-pie
-LDLIBS=`pkg-config --libs vchan-$(BACKEND_VMM)` -lqrexec-utils
+CC ?= gcc
+CFLAGS += -I. -g -O2 -Wall -Wextra -Werror -pie -fPIC `pkg-config --cflags vchan-$(BACKEND_VMM)`
+LDFLAGS += -pie
+LDLIBS = `pkg-config --libs vchan-$(BACKEND_VMM)` -lqrexec-utils
 
 all: qrexec-agent qrexec-client-vm qrexec-fork-server
 qrexec-agent: qrexec-agent.o qrexec-agent-data.o

rpm_spec/core-vm.spec

@@ -205,6 +205,7 @@ ln -sf . %{name}-%{version}
 %setup -T -D
 
 %build
+%{?set_build_flags}
 for dir in qubes-rpc qrexec misc; do
   (cd $dir; make)
 done