DOC: install: recommend pcre2

Makefile comments are also updated to recommend the PCRE2 related
options. PCRE1 is EOL by now:

   https://www.mail-archive.com/haproxy@formilux.org/msg41326.html

(cherry picked from commit 2192dfa6b69f499443afd0f4ffb74eea550d0ecb)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 5916f14d063cfa8fb2f910fb13b8aa069efc5c84)
[wt: trivial ctx adj]
Signed-off-by: Willy Tarreau <w@1wt.eu>
diff --git a/INSTALL b/INSTALL
index 71885e5..03bba74 100644
--- a/INSTALL
+++ b/INSTALL
@@ -38,18 +38,18 @@
   - recent Linux system with all options, make and install :
     $ make clean
     $ make -j $(nproc) TARGET=linux-glibc \
-                USE_OPENSSL=1 USE_LUA=1 USE_PCRE=1 USE_SYSTEMD=1
+                USE_OPENSSL=1 USE_LUA=1 USE_PCRE2=1 USE_SYSTEMD=1
     $ sudo make install
 
   - FreeBSD and OpenBSD, build with all options :
-    $ gmake -j 4 TARGET=freebsd USE_OPENSSL=1 USE_LUA=1 USE_PCRE=1
+    $ gmake -j 4 TARGET=freebsd USE_OPENSSL=1 USE_LUA=1 USE_PCRE2=1
 
   - embedded Linux, build using a cross-compiler :
-    $ make -j $(nproc) TARGET=linux-glibc USE_OPENSSL=1 USE_PCRE=1 \
+    $ make -j $(nproc) TARGET=linux-glibc USE_OPENSSL=1 USE_PCRE2=1 \
                 CC=/opt/cross/gcc730-arm/bin/gcc ADDLIB=-latomic
 
   - Build with static PCRE on Solaris / UltraSPARC :
-    $ make TARGET=solaris CPU=ultrasparc USE_STATIC_PCRE=1
+    $ make TARGET=solaris CPU=ultrasparc USE_STATIC_PCRE2=1
 
 For more advanced build options or if a command above reports an error, please
 read the following sections.
@@ -185,9 +185,9 @@
 
 If you plan on importing a particularly heavy configuration involving a lot of
 regex, you may benefit from using some alternative regex implementations such as
-PCRE. HAProxy natively supports PCRE and PCRE2, both in standard and JIT
-flavors (Just In Time). The following options are available depending on the
-library version provided on your system :
+PCRE. HAProxy natively supports PCRE and PCRE2 (recommended), both in standard
+and JIT flavors (Just In Time). The following options are available depending on
+the library version provided on your system :
 
   - "USE_PCRE=1"         : enable PCRE version 1, dynamic linking
   - "USE_STATIC_PCRE=1"  : enable PCRE version 1, static linking