make: allow per-file conditional LTO compile

Makefile.base

@@ -23,7 +23,7 @@ ${DIRS:%=CLEAN--%}:
 	"$(MAKE)" -C ${@:CLEAN--%=%} clean
 
 ifeq ($(strip $(SRC)),)
-    SRC := $(wildcard *.c)
+    SRC := $(filter-out $(SRC_NOLTO), $(wildcard *.c))
 endif
 ifeq ($(strip $(SRCXX)),)
     SRCXX := $(wildcard *.cpp)
@@ -35,7 +35,9 @@ ifeq ($(strip $(ASSMSRC)),)
     ASSMSRC := $(wildcard *.S)
 endif
 
-OBJC    := $(SRC:%.c=$(BINDIR)$(MODULE)/%.o)
+OBJC_LTO    := $(SRC:%.c=$(BINDIR)$(MODULE)/%.o)
+OBJC_NOLTO  := $(SRC_NOLTO:%.c=$(BINDIR)$(MODULE)/%.o)
+OBJC        := $(OBJC_NOLTO) $(OBJC_LTO)
 OBJCXX      := $(SRCXX:%.cpp=$(BINDIR)$(MODULE)/%.o)
 ASMOBJ      := $(ASMSRC:%.s=$(BINDIR)$(MODULE)/%.o)
 ASSMOBJ     := $(ASSMSRC:%.S=$(BINDIR)$(MODULE)/%.o)
@@ -56,6 +58,8 @@ CXXFLAGS = $(filter-out $(CXXUWFLAGS), $(CFLAGS)) $(CXXEXFLAGS)
 
 # compile and generate dependency info
 
+$(OBJC_LTO): CFLAGS+=$(LTOFLAGS)
+
 $(OBJC): $(BINDIR)$(MODULE)/%.o: %.c $(RIOTBUILD_CONFIG_HEADER_C)
 	$(AD)$(CCACHE) $(CC) \
 		-DRIOT_FILE_RELATIVE=\"$(patsubst $(RIOTBASE)/%,%,$(abspath $<))\" \

Makefile.vars

@@ -35,6 +35,7 @@ export AS                    # The assembler.
 export ASFLAGS               # Flags for the assembler.
 export LINK                  # The command used to link the files. Must take the same parameters as GCC, i.e. "ld" won't work.
 export LINKFLAGS             # Flags to supply in the linking step.
+export LTOFLAGS              # extra CFLAGS for compiling with link time optimization
 export OBJCOPY               # The command used to create the HEXFILE.
 export OFLAGS                # The parameter for OBJCOPY, e.g. to strip the debug information.
 export OBJDUMP               # The command used to create the assembly listing.