Minor update to makefile installation instructions

This removes the use of the option `-flto` in the instructions to compile with a Makefile.

The reason for doing that is that currently, `-flto` can cause some tests to break - see [discussion here](#592 (comment)).

Author: gareth-nx

README.md

@@ -184,7 +184,7 @@ make -f Makefile.manual ADD_FYPPFLAGS=-DMAXRANK=4
 
 You can also specify the compiler and compiler-flags by setting the ``FC`` and ``FFLAGS`` environmental variables. Among other things, this facilitates use of compiler optimizations that are not specified in the Makefile.manual defaults.
 ```sh
-make -f Makefile.manual ADD_FYPPFLAGS=-DMAXRANK=4 FC=gfortran FFLAGS="-O3 -flto"
+make -f Makefile.manual ADD_FYPPFLAGS=-DMAXRANK=4 FC=gfortran FFLAGS="-O3"
 ```
 
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