Made compiling with LTO function like in other cores

I also updated the compiling instructions.  The standalone version and
the MSVC solution file no longer exist, and LTO enabled by default.

COMPILING.md

@@ -1,39 +1,3 @@
-### Windows
-
-#### *Standalone*
-
-1) Open the solution in Visual Studio 2019
-2) Set "GUI.NET" as the Startup Project
-3) Compile as Release/x64 or Release/x86
-4) Run Mesen.exe
-
-Note: When loading the the solution in Visual Studio make sure all the projects are loaded successfully.
-Note: If you get an error about the project targeted .NET Framework 4.5 chose to download the missing packages then install ".NET Framework 4.5 targeting pack" from the Visual Studio Installer.
-
-#### *Libretro*
-
-1) Open the solution in Visual Studio 2019
-2) Compile as Libretro/x64 or Libretro/x86
-3) Use the "mesen_libretro.dll" file in bin/(x64 or x86)/Libretro/mesen_libretro.dll
-
-Note: It's also possible to build the Libretro core via MINGW by using the makefile in the Libretro subfolder.
-
-### Linux
-
-#### *Standalone*
-
-To compile Mesen under Linux you will need clang 7.0+ or gcc 9.0+ (Mesen requires a C++17 compiler with support for the filesystem API.) Additionally, Mesen has the following dependencies:
-
-* Mono 5.18+  (package: mono-devel) 
-* SDL2  (package: libsdl2-dev)
-
-**Note:** **Mono 5.18 or higher is recommended**, some older versions of Mono (e.g 4.2.2) have some stability and performance issues which can cause crashes and slow down the UI.
-The default Mono version in Ubuntu 18.04 is 4.6.2 (which also causes some layout issues in Mesen).  To install the latest version of Mono, follow the instructions here: https://www.mono-project.com/download/stable/#download-lin
-
-The makefile contains some more information at the top.  Running "make" will build the x64 version by default, and then "make run" should start the emulator.
-LTO is supported under clang, which gives a large performance boost (25-30%+), so turning it on is highly recommended (see makefile for details).
-
-#### *Libretro*
-
 To compile the Libretro core you will need a version of clang/gcc that supports C++14.
 Run "make" from the "Libretro" subfolder to build the Libretro core.
+LTO gives a large performance boost (25-30%+), so turning it off is not recommended.

Libretro/Makefile

@@ -88,8 +88,9 @@ else ifeq ($(platform), classic_armv7_a7)
 	TARGET := $(TARGET_NAME)_libretro.so
 	fpic := -fPIC -pthread
 	SHARED := -shared -Wl,--version-script=$(LIBRETRO_DIR)/link.T -Wl,--no-undefined
+	LTO ?= -flto=4
 	CFLAGS += -Ofast \
-	-flto=4 -fwhole-program -fuse-linker-plugin \
+	-fwhole-program -fuse-linker-plugin \
 	-fdata-sections -ffunction-sections -Wl,--gc-sections \
 	-fno-stack-protector -fno-ident -fomit-frame-pointer \
 	-falign-functions=1 -falign-jumps=1 -falign-loops=1 \
@@ -302,11 +303,14 @@ endif
 
 LDFLAGS += $(LIBM)
 
+LTO ?= -flto
+# Backwards compatibility (older versions took true or false rather than a compiler option)
 ifeq ($(LTO),true)
-   CFLAGS += -flto
-   CXXFLAGS += -flto
-   LD = g++
+   override LTO = -flto
+else ifeq ($(LTO),false)
+   override LTO = -fno-lto
 endif
+   LD = $(CXX)
 
 ifeq ($(DEBUG), 1)
   ifneq (,$(findstring msvc,$(platform)))
@@ -359,8 +363,9 @@ ifeq (,$(findstring windows_msvc2017,$(platform)))
   CXXFLAGS += -Wall
 endif
 
-CFLAGS   += -D LIBRETRO $(fpic)
-CXXFLAGS += -D LIBRETRO $(fpic) -std=c++11
+CFLAGS   += -D LIBRETRO $(fpic) $(LTO)
+CXXFLAGS += -D LIBRETRO $(fpic) -std=c++11 $(LTO)
+LDFLAGS  += $(LTO)
 
 all: $(TARGET)