build: Only use *_FOR_BUILD envvars when cross-compiling

This isn't pretty, but it will have to do for now.

Makefile.macos.mk

@@ -343,9 +343,15 @@ build/frida-ios-universal/usr/lib/frida/frida-gadget.dylib: \
 
 define make-ios-env-rule
 build/frida-env-ios-$1.rc: releng/setup-env.sh build/frida-version.h
-	@for os_arch in $$(build_os_arch) ios-$1; do \
+	@if [ $1 != $$(build_os_arch) ]; then \
+		cross=yes; \
+	else \
+		cross=no; \
+	fi; \
+	for os_arch in $$(build_os_arch) ios-$1; do \
 		if [ ! -f build/frida-env-$$$$os_arch.rc ]; then \
 			FRIDA_HOST=$$$$os_arch \
+			FRIDA_CROSS=$$$$cross \
 			FRIDA_PREFIX="$$(abspath build/frida-ios-$1/usr)" \
 			FRIDA_ASAN=$$(FRIDA_ASAN) \
 			XCODE11="$$(XCODE11)" \

Makefile.sdk.mk

@@ -117,9 +117,15 @@ $(eval $(call make-package-rules,$(packages),fs))
 
 
 build/fs-env-%.rc:
-	@for os_arch in $(build_os_arch) $*; do \
+	@if [ $* != $(build_os_arch) ]; then \
+		cross=yes; \
+	else \
+		cross=no; \
+	fi; \
+	for os_arch in $(build_os_arch) $*; do \
 		if [ ! -f build/fs-env-$$os_arch.rc ]; then \
 			FRIDA_HOST=$$os_arch \
+			FRIDA_CROSS=$$cross \
 			FRIDA_ASAN=$(FRIDA_ASAN) \
 			FRIDA_ENV_NAME=fs \
 			FRIDA_ENV_SDK=none \

Makefile.toolchain.mk

@@ -158,16 +158,21 @@ build/ft-%/manifest/ninja.pkg: build/ft-env-%.rc deps/.ninja-stamp
 
 
 build/ft-env-%.rc: build/ft-executable.symbols build/ft-executable.version
-	@for os_arch in $(build_os_arch) $*; do \
+	@if [ $* != $(build_os_arch) ]; then \
+		cross=yes; \
+	else \
+		cross=no; \
+	fi; \
+	for os_arch in $(build_os_arch) $*; do \
 		if [ ! -f build/ft-env-$$os_arch.rc ]; then \
-			FRIDA_HOST=$$os_arch $(frida_env_config) ./releng/setup-env.sh; \
+			FRIDA_HOST=$$os_arch FRIDA_CROSS=$$cross $(frida_env_config) ./releng/setup-env.sh; \
 			case $$? in \
 				0) \
 					;; \
 				2) \
 					if [ "$$os_arch" = "$(build_os_arch)" ]; then \
 						MAKE=$(MAKE) ./releng/bootstrap-toolchain.sh $$os_arch || exit 1; \
-						FRIDA_HOST=$$os_arch $(frida_env_config) ./releng/setup-env.sh || exit 1; \
+						FRIDA_HOST=$$os_arch FRIDA_CROSS=$$cross $(frida_env_config) ./releng/setup-env.sh || exit 1; \
 					else \
 						exit 1; \
 					fi \

releng/frida.mk

@@ -42,18 +42,30 @@ frida_tools = \
 	$(NULL)
 
 build/frida-env-%.rc: releng/setup-env.sh build/frida-version.h
-	@for os_arch in $(build_os_arch) $*; do \
+	@if [ $* != $(build_os_arch) ]; then \
+		cross=yes; \
+	else \
+		cross=no; \
+	fi; \
+	for os_arch in $(build_os_arch) $*; do \
 		if [ ! -f build/frida-env-$$os_arch.rc ]; then \
 			FRIDA_HOST=$$os_arch \
+			FRIDA_CROSS=$$cross \
 			FRIDA_ASAN=$(FRIDA_ASAN) \
 			XCODE11="$(XCODE11)" \
 			./releng/setup-env.sh || exit 1; \
 		fi \
 	done
 build/frida_thin-env-%.rc: releng/setup-env.sh build/frida-version.h
-	@for os_arch in $(build_os_arch) $*; do \
+	@if [ $* != $(build_os_arch) ]; then \
+		cross=yes; \
+	else \
+		cross=no; \
+	fi; \
+	for os_arch in $(build_os_arch) $*; do \
 		if [ ! -f build/frida_thin-env-$$os_arch.rc ]; then \
 			FRIDA_HOST=$$os_arch \
+			FRIDA_CROSS=$$cross \
 			FRIDA_ASAN=$(FRIDA_ASAN) \
 			FRIDA_ENV_NAME=frida_thin \
 			XCODE11="$(XCODE11)" \
@@ -67,9 +79,15 @@ build/frida_thin-env-%.rc: releng/setup-env.sh build/frida-version.h
 	[ ! -d toolchain-$* ] && ln -s frida_thin-toolchain-$* toolchain-$*; \
 	true
 build/frida_gir-env-%.rc: releng/setup-env.sh build/frida-version.h
-	@for os_arch in $(build_os_arch) $*; do \
+	@if [ $* != $(build_os_arch) ]; then \
+		cross=yes; \
+	else \
+		cross=no; \
+	fi; \
+	for os_arch in $(build_os_arch) $*; do \
 		if [ ! -f build/frida_gir-env-$$os_arch.rc ]; then \
 			FRIDA_HOST=$$os_arch \
+			FRIDA_CROSS=$$cross \
 			FRIDA_ASAN=$(FRIDA_ASAN) \
 			FRIDA_ENV_NAME=frida_gir \
 			XCODE11="$(XCODE11)" \

releng/setup-env.sh

@@ -52,7 +52,7 @@ read_toolchain_variable ()
   local var_name=$1
   local fallback=$2
 
-  if [ $host_os_arch == $build_os_arch ]; then
+  if [ $host_os_arch == $build_os_arch ] && [ "$FRIDA_CROSS" == yes ]; then
     local contextual_var_name=${var_name}_FOR_BUILD
   else
     local contextual_var_name=${var_name}