fix Issue 17966 - don't build libphobos2.a with PIC for i386

- use MODEL=64/32 to decide on default
- allow overriding default with explicit PIC=0/1 make args

Author: MartinNowak

posix.mak

@@ -29,9 +29,23 @@ INSTALL_DIR=../install
 DOCDIR=doc
 IMPDIR=import
 
-OPTIONAL_PIC:=$(if $(PIC),-fPIC,)
 OPTIONAL_COVERAGE:=$(if $(TEST_COVERAGE),-cov,)
 
+# default to PIC on x86_64, use PIC=1/0 to en-/disable PIC.
+# Note that shared libraries and C files are always compiled with PIC.
+ifeq ($(PIC),)
+    ifeq ($(MODEL),64) # x86_64
+        PIC:=1
+    else
+        PIC:=0
+    endif
+endif
+ifeq ($(PIC),1)
+    override PIC:=-fPIC
+else
+    override PIC:=
+endif
+
 ifeq (osx,$(OS))
 	DOTDLL:=.dylib
 	DOTLIB:=.a
@@ -55,7 +69,7 @@ ifeq (solaris,$(OS))
 endif
 
 # Set DFLAGS
-UDFLAGS:=-conf= -Isrc -Iimport -w -dip1000 $(MODEL_FLAG) $(OPTIONAL_PIC) $(OPTIONAL_COVERAGE)
+UDFLAGS:=-conf= -Isrc -Iimport -w -dip1000 $(MODEL_FLAG) $(PIC) $(OPTIONAL_COVERAGE)
 ifeq ($(BUILD),debug)
 	UDFLAGS += -g -debug
 	DFLAGS:=$(UDFLAGS)

test/common.mak

@@ -14,13 +14,11 @@ SRC:=src
 GENERATED:=./generated
 ROOT:=$(GENERATED)/$(OS)/$(BUILD)/$(MODEL)
 
-OPTIONAL_PIC:=$(if $(PIC),-fPIC,)
-
 ifneq (default,$(MODEL))
 	MODEL_FLAG:=-m$(MODEL)
 endif
-CFLAGS:=$(MODEL_FLAG) -Wall
-DFLAGS:=$(MODEL_FLAG) -w -I../../src -I../../import -I$(SRC) -defaultlib= -debuglib= -dip1000 $(OPTIONAL_PIC)
+CFLAGS:=$(MODEL_FLAG) $(PIC) -Wall
+DFLAGS:=$(MODEL_FLAG) $(PIC) -w -I../../src -I../../import -I$(SRC) -defaultlib= -debuglib= -dip1000
 # LINK_SHARED may be set by importing makefile
 DFLAGS+=$(if $(LINK_SHARED),-L$(DRUNTIMESO),-L$(DRUNTIME))
 ifeq ($(BUILD),debug)