Change CI scripts to use "autoreconf -i" and update docs

autoconf 2.70 requires use of `autoreconf -i` for the
AC_CANONICAL_HOST macro, which is pulled in by AC_FUNC_MMAP.

Update CI configuration to use this, and change the instructions in
the INSTALL and README.md files to match.

Author: daviesrob

.appveyor.yml

@@ -34,7 +34,7 @@ build_script:
   - set MSYSTEM=MINGW64
   - set PATH=C:/msys64/usr/bin;C:/msys64/mingw64/bin;%PATH%
   - git submodule update --init --recursive
-  - "sh -lc \"autoheader && autoconf && ./configure --enable-werror CFLAGS='-g -O3' && make -j2\""
+  - "sh -lc \"autoreconf -i && ./configure --enable-werror CFLAGS='-g -O3' && make -j2\""
 
 #build_script:
 #  - make

.cirrus.yml

@@ -33,7 +33,7 @@ compile_template: &COMPILE
     fi
     if test "$USE_CONFIG" = "yes"; then
       MAKE_OPTS=
-      autoreconf
+      autoreconf -i
       eval ./configure --enable-plugins --enable-werror $CONFIG_OPTS CFLAGS=\"-g -O3 $CFLAGS\" || \
         ( cat config.log; false )
     else

INSTALL

@@ -17,6 +17,7 @@ In addition, building the configure script requires:
 
     autoheader
     autoconf
+    autoreconf
 
 Running the configure script uses awk, along with a number of
 standard UNIX tools (cat, cp, grep, mv, rm, sed, among others).  Almost
@@ -79,12 +80,7 @@ This step is only needed if configure.ac has been changed, or if configure
 does not exist (for example, when building from a git clone).  The
 configure script and config.h.in can be built by running:
 
-    autoheader
-    autoconf
-
-If you have a full GNU autotools install, you can alternatively run:
-
-    autoreconf
+    autoreconf -i
 
 Basic Installation
 ==================

README.md

@@ -28,8 +28,7 @@ committed to this repository, so building the code from a Git repository
 requires extra steps:
 
 ```sh
-autoheader     # If using configure, generate the header template...
-autoconf       # ...and configure script (or use autoreconf to do both)
+autoreconf -i  # Build the configure script and install files it uses
 ./configure    # Optional but recommended, for choosing extra functionality
 make
 make install