makefile: Add explicit option LTO=1 to enable Link Time Optimization builds

When building compiler optimized binaries with the gcc or clang compilers,
invoking GNU make with LTO=1 on the command line will cause the build
to use Link Time Optimization to maximally optimize the results.
Link Time Optimization can report errors which aren't otherwise detected
and will also take significantly longer to complete.

makefile

@@ -44,6 +44,13 @@
 # If debugging is desired, then GNU make can be invoked with
 # DEBUG=1 on the command line.
 #
+# When building compiler optimized binaries with the gcc or clang 
+# compilers, invoking GNU make with LTO=1 on the command line will 
+# cause the build to use Link Time Optimization to maximally optimize 
+# the results.  Link Time Optimization can report errors which aren't 
+# otherwise detected and will also take significantly longer to 
+# complete.
+#
 # The default build will run per simulator tests if they are 
 # available.  If building without running tests is desired, 
 # then GNU make should be invoked with TESTS=0 on the command 
@@ -485,7 +492,7 @@ ifeq (${WIN32},)  #*nix Environments (&& cygwin)
                 endif
                 OS_CCDEFS += -D_HPUX_SOURCE -D_LARGEFILE64_SOURCE
                 OS_LDFLAGS += -Wl,+b:
-                NO_LTO = 1
+                LTO =
               else
                 LIBEXT = a
               endif
@@ -509,14 +516,6 @@ ifeq (${WIN32},)  #*nix Environments (&& cygwin)
     endif
     export CPATH = $(subst $() $(),:,$(INCPATH))
     export LIBRARY_PATH = $(subst $() $(),:,$(LIBPATH))
-    # Some gcc versions don't support LTO, so only use LTO when the compiler is known to support it
-    ifeq (,$(NO_LTO))
-      ifneq (,$(GCC_VERSION))
-        ifeq (,$(shell ${GCC} -v /dev/null 2>&1 | grep '\-\-enable-lto'))
-          LTO_EXCLUDE_VERSIONS += $(GCC_VERSION)
-        endif
-      endif
-    endif
   endif
   $(info lib paths are: ${LIBPATH})
   $(info include paths are: ${INCPATH})
@@ -1268,18 +1267,13 @@ ifneq ($(DEBUG),)
   CFLAGS_G = -g -ggdb -g3
   CFLAGS_O = -O0
   BUILD_FEATURES = - debugging support
+  LTO =
 else
   ifneq (,$(findstring clang,$(COMPILER_NAME))$(findstring LLVM,$(COMPILER_NAME)))
     CFLAGS_O = -O2 -fno-strict-overflow
-    GCC_OPTIMIZERS_CMD = ${GCC} --help
+    GCC_OPTIMIZERS_CMD = ${GCC} --help 2>&1
-    NO_LTO = 1
   else
-    NO_LTO = 1
+    CFLAGS_O := -O2
-    ifeq (Darwin,$(OSTYPE))
-      CFLAGS_O += -O4 -flto -fwhole-program
-    else
-      CFLAGS_O := -O2
-    endif
   endif
   LDFLAGS_O = 
   GCC_MAJOR_VERSION = $(firstword $(subst  ., ,$(GCC_VERSION)))
@@ -1295,9 +1289,6 @@ else
   ifneq (,$(GCC_COMMON_CMD))
     GCC_OPTIMIZERS += $(shell $(GCC_COMMON_CMD))
   endif
-  ifneq (,$(findstring $(GCC_VERSION),$(LTO_EXCLUDE_VERSIONS)))
-    NO_LTO = 1
-  endif
   ifneq (,$(findstring -finline-functions,$(GCC_OPTIMIZERS)))
     CFLAGS_O += -finline-functions
   endif
@@ -1316,13 +1307,19 @@ else
   ifneq (,$(findstring -fstrict-overflow,$(GCC_OPTIMIZERS)))
     CFLAGS_O += -fno-strict-overflow
   endif
-  ifeq (,$(NO_LTO))
+  ifneq (,$(findstring $(GCC_VERSION),$(LTO_EXCLUDE_VERSIONS)))
+    LTO =
+  endif
+  ifneq (,$(LTO))
     ifneq (,$(findstring -flto,$(GCC_OPTIMIZERS)))
-      CFLAGS_O += -flto -fwhole-program
+      CFLAGS_O += -flto
-      LDFLAGS_O += -flto -fwhole-program
+      ifneq (,$(and $(findstring gcc,$(COMPILER_NAME)),$(findstring -fwhole-program,$(GCC_OPTIMIZERS))))
+        CFLAGS_O += -fwhole-program
+      endif
+      LTO_FEATURE = , with Link Time Optimization,
     endif
   endif
-  BUILD_FEATURES = - compiler optimizations and no debugging support
+  BUILD_FEATURES = - compiler optimizations$(LTO_FEATURE) and no debugging support
 endif
 ifneq (3,$(GCC_MAJOR_VERSION))
   ifeq (,$(GCC_WARNINGS_CMD))