makefile: honor build env CFLAGS and LDFLAGS

Generally build env (rpmbuild, dpkg-buildpackage) provide sane and
useful defaults.
Fixes hardening-no-relro lintian warning.

QubesOS/qubes-issues#1416

qmemman/Makefile

@@ -1,5 +1,5 @@
 CC=gcc
-CFLAGS=-Wall -Wextra -Werror -g -O3
+CFLAGS+=-Wall -Wextra -Werror -g -O3
 all:	meminfo-writer
 SBINDIR?=/usr/sbin
 
@@ -9,7 +9,7 @@ ifneq "$(_XENSTORE_H)" ""
 endif
 
 meminfo-writer: meminfo-writer.o
-	$(CC) -g -o meminfo-writer meminfo-writer.o -lxenstore
+	$(CC) $(LDFLAGS) -g -o meminfo-writer meminfo-writer.o -lxenstore
 install:
 	install -D meminfo-writer $(DESTDIR)/$(SBINDIR)/meminfo-writer
 ifeq (1,${DEBIANBUILD})

qrexec-lib/Makefile

@@ -2,7 +2,7 @@ CC=gcc
 CFLAGS+=-I. -g -O2 -Wall -Wextra -Werror -pie -fPIC `pkg-config --cflags vchan-$(BACKEND_VMM)`
 COMMONIOALL=ioall.o
 SO_VER=2
-LDFLAGS=-shared
+LDFLAGS+=-shared
 VCHANLIBS = `pkg-config --libs vchan-$(BACKEND_VMM)`
 
 _XENSTORE_H=$(shell ls /usr/include/xenstore.h)