[LibOS,Pal] Makefiles: use `+=` instead of `=` in CFLAGS/LDFLAGS/etc

LibOS/shim/src/Makefile

@@ -4,7 +4,7 @@ include ../../../Scripts/Makefile.configs
 
 OMIT_FRAME_POINTER = no
 
-CFLAGS = -Wall -fPIC -std=c11 -Winline -Wwrite-strings \
+CFLAGS += -Wall -fPIC -std=c11 -Winline -Wwrite-strings \
 	  -fmerge-all-constants -Wstrict-prototypes -Wtrampolines \
 	  -Werror=implicit-function-declaration \
 	  $(cc-option, -Wnull-dereference) \
@@ -21,9 +21,9 @@ CFLAGS += -DOMIT_FRAME_POINTER=1
 else
 CFLAGS += -fno-omit-frame-pointer -DOMIT_FRAME_POINTER=0
 endif
-ASFLAGS	= -Wa,--noexecstack -x assembler-with-cpp -I../include
+ASFLAGS += -Wa,--noexecstack -x assembler-with-cpp -I../include
 
-LDFLAGS	= -shared -nostdlib -z combreloc -z relro -z now -z defs \
+LDFLAGS += -shared -nostdlib -z combreloc -z relro -z now -z defs \
 	  -dynamic-link=libpal.so \
 	  -rpath-link=$(abspath $(RUNTIME_DIR))
 

Pal/regression/Makefile

@@ -1,7 +1,7 @@
 include ../../Scripts/Makefile.configs
 include ../../Scripts/Makefile.rules
 
-CFLAGS	= -Wall -O2 -std=c11 -fno-builtin -nostdlib \
+CFLAGS	+= -Wall -O2 -std=c11 -fno-builtin -nostdlib \
 	  -I../include/pal -I../lib -I../src
 
 preloads = \

Pal/src/host/Linux-SGX/Makefile.am

@@ -1,15 +1,15 @@
 # Add host-specific compilation rules here
 
-CFLAGS	= -Wall -fPIC -O2 -maes -std=c11 -U_FORTIFY_SOURCE \
+CFLAGS += -Wall -fPIC -O2 -maes -std=c11 -U_FORTIFY_SOURCE \
 	  -fno-stack-protector -fno-builtin -Wtrampolines
 
 EXTRAFLAGS = -Wextra $(call cc-option,-Wnull-dereference)
 
 CFLAGS += $(EXTRAFLAGS)
 
-ASFLAGS = -DPIC -DSHARED -fPIC -DASSEMBLER -Wa,--noexecstack \
+ASFLAGS += -DPIC -DSHARED -fPIC -DASSEMBLER -Wa,--noexecstack \
 	  -x assembler-with-cpp
-LDFLAGS	= -shared -nostdlib -z combreloc -z defs \
+LDFLAGS += -shared -nostdlib -z combreloc -z defs \
 	  --version-script $(HOST_DIR)/pal.map -T $(HOST_DIR)/enclave.lds \
 	  --hash-style=gnu -z relro -z now
 

Pal/src/host/Linux/Makefile.am

@@ -1,16 +1,16 @@
 # Add host-specific compilation rules here
 SEC_DIR = security/$(PAL_HOST)
 
-CFLAGS	= -Wall -fPIC -O2 -std=c11 -U_FORTIFY_SOURCE \
+CFLAGS	+= -Wall -fPIC -O2 -std=c11 -U_FORTIFY_SOURCE \
 	  -fno-stack-protector -fno-builtin -Wtrampolines
 
 EXTRAFLAGS = -Wextra $(call cc-option,-Wnull-dereference)
 
 CFLAGS += $(EXTRAFLAGS)
 
-ASFLAGS = -DPIC -DSHARED -fPIC -DASSEMBLER -Wa,--noexecstack \
+ASFLAGS += -DPIC -DSHARED -fPIC -DASSEMBLER -Wa,--noexecstack \
 	  -x assembler-with-cpp
-LDFLAGS	= -shared -nostdlib -z combreloc -z defs \
+LDFLAGS += -shared -nostdlib -z combreloc -z defs \
 	  --version-script $(HOST_DIR)/pal.map -T $(HOST_DIR)/pal.lds \
 	  -z relro -z now
 

Pal/src/host/Skeleton/Makefile.am

@@ -1,15 +1,13 @@
 # Add host-specific compilation rules here
 
-CFLAGS = -Wall -fPIC -O2 -std=c11 -U_FORTIFY_SOURCE \
+CFLAGS += -Wall -fPIC -O2 -std=c11 -U_FORTIFY_SOURCE \
          -fno-stack-protector -fno-builtin
 
-EXTRAFLAGS = -Wextra -Wno-unused-parameter -Wno-sign-compare $(call cc-option,-Wnull-dereference)
+CFLAGS += -Wextra -Wno-unused-parameter -Wno-sign-compare $(call cc-option,-Wnull-dereference)
 
-CFLAGS += $(EXTRAFLAGS)
-
-ASFLAGS = -DPIC -DSHARED -fPIC -DASSEMBLER -Wa,--noexecstack \
+ASFLAGS += -DPIC -DSHARED -fPIC -DASSEMBLER -Wa,--noexecstack \
 	  -x assembler-with-cpp
-LDFLAGS = -shared -nostdlib -z combreloc -z defs \
+LDFLAGS += -shared -nostdlib -z combreloc -z defs \
 	  --version-script $(HOST_DIR)/pal.map -T $(HOST_DIR)/pal.lds
 
 ifeq ($(WERROR),1)

Pal/test/Makefile

@@ -1,7 +1,7 @@
 include ../../Scripts/Makefile.configs
 include ../../Scripts/Makefile.rules
 
-CFLAGS	= -Wall -O2 -std=c11 -fno-builtin -nostdlib -no-pie \
+CFLAGS += -Wall -O2 -std=c11 -fno-builtin -nostdlib -no-pie \
 	  -I../include/pal -I../lib
 
 .PHONY: default
@@ -45,7 +45,7 @@ crt_init-recurse:
 	@true
 
 CFLAGS-Pie = -fPIC -pie
-LDLIBS = $(graphene_lib) $(pal_lib) ../crt_init/user_start.o
+LDLIBS += $(graphene_lib) $(pal_lib) ../crt_init/user_start.o
 $(executables): %: %.c $(LDLIBS)
 	$(call cmd,csingle)
 

Scripts/Makefile.Test

@@ -15,9 +15,9 @@ else
 PAL_LOADER = pal_loader
 endif
 
-CFLAGS 	= -Wall -std=gnu99
-CXXFLAGS = -Wall -std=c++14
-LDFLAGS	=
+CFLAGS += -Wall -std=gnu99
+CXXFLAGS += -Wall -std=c++14
+LDFLAGS	+=
 
 CFLAGS-libos = -I$(SHIMDIR)/include -L$(SHIMDIR)/../glibc-build/libos