Arduino.mk: make LTO optional, enabled by default

On Fedora 25 with avr-gcc 6.2.0 LTO crashes, e.g., if Serial is used.

Arduino.mk

@@ -1039,9 +1039,17 @@ ifndef AR_NAME
     endif
 endif
 
+ifeq ($(shell expr $(CC_VERNUM) '>' 490), 1)
+    USE_LTO ?= 1
+endif
+
+
 ifndef CFLAGS_STD
     ifeq ($(shell expr $(CC_VERNUM) '>' 490), 1)
-        CFLAGS_STD      = -std=gnu11 -flto -fno-fat-lto-objects
+        CFLAGS_STD      = -std=gnu11
+        ifeq ($(USE_LTO),1)
+            CFLAGS_STD += -flto -fno-fat-lto-objects
+        endif
     else
         CFLAGS_STD        =
     endif
@@ -1052,7 +1060,10 @@ endif
 
 ifndef CXXFLAGS_STD
     ifeq ($(shell expr $(CC_VERNUM) '>' 490), 1)
-        CXXFLAGS_STD      = -std=gnu++11 -fno-threadsafe-statics -flto
+        CXXFLAGS_STD      = -std=gnu++11 -fno-threadsafe-statics
+        ifeq ($(USE_LTO),1)
+            CXXFLAGS_STD += -flto
+        endif
     else
         CXXFLAGS_STD      =
     endif
@@ -1064,11 +1075,11 @@ endif
 CFLAGS        += $(CFLAGS_STD)
 CXXFLAGS      += -fpermissive -fno-exceptions $(CXXFLAGS_STD)
 ASFLAGS       += -x assembler-with-cpp
-ifeq ($(shell expr $(CC_VERNUM) '>' 490), 1)
+ifeq ($(USE_LTO), 1)
     ASFLAGS += -flto
 endif
 LDFLAGS       += -$(MCU_FLAG_NAME)=$(MCU) -Wl,--gc-sections -O$(OPTIMIZATION_LEVEL)
-ifeq ($(shell expr $(CC_VERNUM) '>' 490), 1)
+ifeq ($(USE_LTO), 1)
     LDFLAGS += -flto -fuse-linker-plugin
 endif
 SIZEFLAGS     ?= --mcu=$(MCU) -C

HISTORY.md

@@ -7,6 +7,7 @@ I tried to give credit whenever possible. If I have missed anyone, kindly add it
 ### In Development
 - Fix: Do not include the Arduino header when calling generate_assembly on .cpp files. (https://github.com/Batchyx)
 - Fix: Auto-detect F_CPU on Teensy from boards.txt (https://github.com/DaWelter)
+- Tweak: Allow to disable LTO (currently crashes on Fedora 25)
 
 ### 1.5.2 (2017-01-11)