README.md: Merge discussion of compilers into Installation Manual, remove from README.md

Author: Matthias Koeppe

README.md

@@ -298,7 +298,8 @@ Guide](https://doc.sagemath.org/html/en/installation).
    `./configure --help`).
 
 13. Type `make`.  That's it! Everything is automatic and
-   non-interactive; but it will a few hours (on a recent computer).
+   non-interactive; but it will take a few hours (on a recent
+   computer).
 
    The build should work fine on all fully supported platforms. If it
    does not, we want to know!
@@ -345,28 +346,6 @@ messages in them.  Send (a small part of) the relevant log file to the
 making sure to include at least some of the error messages; probably
 someone there will have some helpful suggestions.
 
-Supported Compilers
--------------------
-
-Sage includes a GCC (_GNU Compiler Collection_) package. However,
-it almost always better to use C, C++ and Fortran compilers
-already available on the system. To force using  specific compilers,
-set environment variables `CC`, `CXX`, and `FC` (for C, C++, and Fortran compilers,
-respectively) to the desired values,
-and run `./configure`. E.g. `CC=clang CXX=clang++ FC=gfortran ./configure`
-will configure Sage to be built with Clang C/C++ compilers and Fortran
-compiler gfortran.
-
-It is determined automatically whether Sage's GCC package, or just its part containing
-Fortran compiler `gfortran` needs to be installed. This can be overwritten
-by running `./configure` with option `--without-system-gcc`.
-
-There are some known problems with old assemblers, in particular when
-building the ECM package. You should ensure that your assembler
-understands all instructions for your processor. On Linux, this means
-you need a recent version of binutils; on MacOS you need a recent version
-of Xcode.
-
 Contributing to Sage
 --------------------
 

src/doc/en/installation/source.rst

@@ -121,7 +121,13 @@ development files.
 Fortran and compiler suites
 ###########################
 
-Sage installation also needs a Fortran compiler. Officially we support
+Sage installation also needs a Fortran compiler.  It is determined
+automatically whether Sage's GCC package, or just its part containing
+Fortran compiler `gfortran` needs to be installed. This can be
+overwritten by running `./configure` with option
+`--without-system-gcc`.
+
+Officially we support
 gfortran from `GNU Compiler Collection (GCC) <https://gcc.gnu.org/>`_.
 If C and C++ compilers also come from there (i.e., gcc and g++), their versions
 should match.
@@ -136,6 +142,13 @@ this is work in progress at the moment (May 2019)).
 Therefore, if you plan on using your own GCC compilers, then make sure that
 their versions match.
 
+To force using specific compilers, set environment variables ``CC``,
+``CXX``, and ``FC`` (for C, C++, and Fortran compilers, respectively)
+to the desired values, and run ``./configure``. For example,
+``./configure CC=clang CXX=clang++ FC=gfortran`` will configure Sage
+to be built with Clang C/C++ compilers and Fortran compiler
+``gfortran``.
+
 Alternatively, Sage includes a GCC package, so that C, C++ and Fortran
 compilers will be built when the build system detects that it is needed,
 e.g., non-GCC compilers, or
@@ -148,6 +161,13 @@ Note that you can always override this behavior through the configure
 options ``--without-system-gcc`` and ``--with-system-gcc``, see
 :ref:`section_compilers`.
 
+There are some known problems with old assemblers, in particular when
+building the ``ecm`` and ``fflas_ffpack`` packages. You should ensure
+that your assembler understands all instructions for your
+processor. On Linux, this means you need a recent version of
+``binutils``; on macOS you need a recent version of Xcode.
+
+
 Other notes
 ^^^^^^^^^^^