DOC: update INSTALL to reflect the minimum compiler version

The mt_list update in 3.1 mandated the support for c11-like atomics that
arrived with gcc-4.7. As such, older versions are no longer supported.
For special cases in single-threaded environments, mt_lists could be
replaced with regular lists but it doesn't seem worth the hassle. It
was verified that gcc 4.7 to 14 and clang 3.0 and 19 do build fine.
That leaves us with 10 years of coverage of compiler versions, which
remains reasonable assuming that users of old ultra-stable systems are
unlikely to upgrade haproxy without touching the rest of the system.

This should be backported to 3.1.

INSTALL

@@ -111,20 +111,22 @@ HAProxy requires a working GCC or Clang toolchain and GNU make :
     may want to retry with "gmake" which is the name commonly used for GNU make
     on BSD systems.
 
-  - GCC >= 4.2 (up to 14 tested). Older versions can be made to work with a
-    few minor adaptations if really needed. Newer versions may sometimes break
-    due to compiler regressions or behaviour changes. The version shipped with
-    your operating system is very likely to work with no trouble. Clang >= 3.0
-    is also known to work as an alternative solution. Recent versions may emit
-    a bit more warnings that are worth reporting as they may reveal real bugs.
-    TCC (https://repo.or.cz/tinycc.git) is also usable for developers but will
-    not support threading and was found at least once to produce bad code in
-    some rare corner cases (since fixed). But it builds extremely quickly
-    (typically half a second for the whole project) and is very convenient to
-    run quick tests during API changes or code refactoring.
+  - GCC >= 4.7 (up to 14 tested). Older versions are no longer supported due to
+    the latest mt_list update which only uses c11-like atomics. Newer versions
+    may sometimes break due to compiler regressions or behaviour changes. The
+    version shipped with your operating system is very likely to work with no
+    trouble. Clang >= 3.0 is also known to work as an alternative solution, and
+    versions up to 19 were successfully tested. Recent versions may emit a bit
+    more warnings that are worth reporting as they may reveal real bugs. TCC
+    (https://repo.or.cz/tinycc.git) is also usable for developers but will not
+    support threading and was found at least once to produce bad code in some
+    rare corner cases (since fixed). But it builds extremely quickly (typically
+    half a second for the whole project) and is very convenient to run quick
+    tests during API changes or code refactoring.
 
   - GNU ld (binutils package), with no particular version. Other linkers might
-    work but were not tested.
+    work but were not tested. The default one from your operating system will
+    normally work.
 
 On debian or Ubuntu systems and their derivatives, you may get all these tools
 at once by issuing the two following commands :
@@ -488,8 +490,8 @@ target. Common issues may include:
        other supported compatible library.
 
   - many "dereferencing pointer 'sa.985' does break strict-aliasing rules"
-    => these warnings happen on old compilers (typically gcc-4.4), and may
-       safely be ignored; newer ones are better on these.
+    => these warnings happen on old compilers (typically gcc before 7.x),
+       and may safely be ignored; newer ones are better on these.
 
 
 4.11) QUIC
@@ -754,8 +756,8 @@ forced to produce final binaries, and must not be used during bisect sessions,
 as it will often lead to the wrong commit.
 
 Examples:
-  # silence strict-aliasing warnings with old gcc-4.4:
-  $ make -j$(nproc) TARGET=linux-glibc CC=gcc-44 CFLAGS=-fno-strict-aliasing
+  # silence strict-aliasing warnings with old gcc-5.5:
+  $ make -j$(nproc) TARGET=linux-glibc CC=gcc-55 CFLAGS=-fno-strict-aliasing
 
   # disable all warning options:
   $ make -j$(nproc) TARGET=linux-glibc CC=mycc WARN_CFLAGS= NOWARN_CFLAGS=