DOC: add notes on GCC 6.5 being the minimum supported GCC version.

Given that GCC 6 is mainly kept for Ubuntu LTS and that has 6.5
when you install gcc-6 with `apt`, let's document that as the
minimum version (also that is what is tested in CI).

[ci skip]

INSTALL.rst.txt

@@ -75,7 +75,8 @@ skipped when running the test suite if no Fortran compiler is available.  For
 building Scipy a Fortran compiler is needed though, so we include some details
 on Fortran compilers in the rest of this section.
 
-On OS X and Linux, all common compilers will work.
+On OS X and Linux, all common compilers will work. The minimum supported GCC
+version is 6.5.
 
 For Fortran, ``gfortran`` works, ``g77`` does not.  In case ``g77`` is
 installed then ``g77`` will be detected and used first.  To explicitly select

doc/source/user/building.rst

@@ -59,7 +59,7 @@ Building NumPy requires the following software installed:
    MSVC and Clang compilers. Compilers from other vendors such as Intel,
    Absoft, Sun, NAG, Compaq, Vast, Portland, Lahey, HP, IBM are only
    supported in the form of community feedback, and may not work out of the
-   box.  GCC 4.x (and later) compilers are recommended. On ARM64 (aarch64)
+   box.  GCC 6.5 (and later) compilers are recommended. On ARM64 (aarch64)
    GCC 8.x (and later) are recommended.
 
 3) Linear Algebra libraries