build: show the compiler and the toolchain in the version banner

The boot banner and the version command print the SDCC release and
revision from the compiler's own macros, and the name of the in-tree
toolchain the Makefile built, which is the checksum of sdcc/ and so
changes with every patch in it. A build with CC given on the command
line says "system" instead, since that compiler need not carry the
patches. A plain release and the same release with the patches report
the same revision, so the toolchain line is what tells them apart.

Costs 66 bytes of BANK2.

Makefile

@@ -4,11 +4,14 @@ DEFAULT_CONFIG_LOCATION = 454656
 CONFIG_LOCATION = 458752
 HTML_LOCATION = 262144
 
-SDCC_PREFIX = toolchain/sdcc-$(shell cat sdcc/* | cksum | cut -d' ' -f1)
+SDCC_TOOLCHAIN = sdcc-$(shell cat sdcc/* | cksum | cut -d' ' -f1)
+SDCC_PREFIX = toolchain/$(SDCC_TOOLCHAIN)
 ifeq ($(origin CC),default)
 SDCC = $(SDCC_PREFIX)/bin/sdcc
 CC = $(SDCC)
 ASM = $(SDCC_PREFIX)/bin/sdas8051
+else
+SDCC_TOOLCHAIN = system
 endif
 CC_FLAGS = -mmcs51 -I. -Imachine -Ihttpd -Iuip
 ASM ?= sdas8051
@@ -172,6 +175,7 @@ $(VERSION_HEADER): FORCE
 	@printf '%s\n' "#ifndef VERSION_H" "#define VERSION_H" \
 		"#define VERSION_SW \"$(VERSION_EXTENSION)\"" \
 		"#define BUILD_DATE \"$(BUILD_DATE)\"" \
+		"#define SDCC_TOOLCHAIN \"$(SDCC_TOOLCHAIN)\"" \
 		"#endif" > $(VERSION_HEADER).tmp
 	@cmp -s $(VERSION_HEADER).tmp $(VERSION_HEADER) \
 		&& rm -f $(VERSION_HEADER).tmp \