Update README.SRC to enable success by newbies.

Need i386 version of zlib, updated libmfpr.so, g++ (not clang++).
https://github.com/upx/upx/issues/938
	modified:   README.SRC

README.SRC

@@ -81,6 +81,15 @@ Developer quick start
  4b)    $ wget https://github.com/upx/upx-stubtools/releases/download/v20221212/bin-upx-20221212.tar.xz
           # Extract to $HOME/bin/bin-upx or $HOME/local/bin/bin-upx.
           # See directions in: https://github.com/upx/upx-stubtools
+
+          # Some of the pre-built tools need i386 libz.  On Ubuntu:
+          #   # dpkg --add-architecture i386
+          #   # apt-get update
+          #   # apt-get install zlib1g-dev
+
+          # aarch64-linux-gnu/4.9.2/cc1 needs libmpfr.so.4.  Make a symlink:
+          #   # ln -s /usr/lib/x86_64-linux-gnu/libmpfr.so.6 /usr/lib/x86_64-linux-gnu/libmpfr.so.4
+
  4c)    $ make clean; make  # verify that you can re-build stubs
  4d)    $ popd  # my-upx
  5)  $ cd upx  # my-upx/upx
@@ -92,7 +101,18 @@ Developer quick start
         # If stuck, then reset via "rm -rf vendor; git submodule update --init".
  7)  $ mkdir -p build/debug build/release
  8)  $ pushd build/debug  # my-upx/upx/build/debug
- 9)  $ cmake ../..  # configure cmake
+
+9a)  Install GNU g++ so that it is used when invoking "c++" compiler.
+     On Ubuntu 24.04 with
+        cmake: cmake version 3.28.3
+        g++: Ubuntu clang version 18.1.3 (1ubuntu1)
+        /lib/llvm-18/lib/cmake/llvm/HandleLLVMOptions.cmake:
+           add_flag_if_supported("-fno-lifetime-dse" CMAKE_CXX_FLAGS)
+     then cmake invoking c++ uses -fno-lifetime-dse but gets
+        c++: error: unknown argument: '-fno-lifetime-dse'
+     Commenting out the "add_flag_if_supported' line still uses that unknown argument.
+     Installing g++ avoids the problem.
+9b)  $ cmake ../..  # configure cmake
 10)  $ cmake --build . --verbose  # optional "--verbose"
         # This relies on stub/*.h, which YOU must re-build if necessary.
         # When in doubt, then use "cd src/stub; make clean; make".