Fix and improve configuring code in makefile and script.

Makefile

@@ -152,7 +152,7 @@ install: docs vimpager.configured vimcat.configured
 	sed  -e '1{ s|.*|#!'"$$POSIX_SHELL"'|; }' \
 	     -e 's|\$${POSIX_SHELL}|'"$$POSIX_SHELL|" \
 	     -e '/^[ 	]*\.[ 	]*.*inc\/prologue.sh.*$$/d' \
-	     -e 's/^[ 	]*\.[ 	]*.*inc\/platform.sh.*$$/'"`scripts/find_platform`"/ \
+	     -e 's|^[ 	]*\.[ 	]*.*inc/platform.sh.*$$|'"`scripts/find_platform`|" \
 	     -e 's|^version=.*|version="'"`git describe`"' (configured, shell='"$$POSIX_SHELL"')"|' \
 	     -e 's|^runtime=.*$$|runtime="${PREFIX}/share/vimpager"|' \
 	     -e 's|^vimcat=.*$$|vimcat="${PREFIX}/bin/vimcat"|' \

scripts/find_platform

@@ -2,20 +2,29 @@
 
 . inc/platform.sh
 
+# The backslash has to be put here 8 times: foctor two to get past the initial
+# shell quoting, another factor two because it is used in the replacement
+# string for sed, another factor two because the output of belows echo will be
+# used in the replacement text in the sed call in the makefile.  This yields
+# one backslash that can escape the single quote in the configured vimpager
+# script.
+AWK=$(echo "$AWK" | sed "s/'/'\\\\\\\\''/g")
+SED=$(echo "$SED" | sed "s/'/'\\\\\\\\''/g")
+
 echo \
-  bsd=$bsd \
-  cygwin=$cygwin \
-  freebsd=$freebsd \
-  linux=$linux \
-  msys=$msys \
-  netbsd=$netbsd \
-  openbsd=$openbsd \
-  osx=$osx \
-  solaris=$solaris \
-  win32=$win32 \
-  AWK=$AWK \
-  SED=$SED \
-  HEAD_SYNTAX=$HEAD_SYNTAX \
-  TAIL_SYNTAX=$TAIL_SYNTAX \
-  TAIL_NO_DOUBLE_DASH=$TAIL_NO_DOUBLE_DASH \
+  ${bsd:+bsd=$bsd} \
+  ${cygwin:+cygwin=$cygwin} \
+  ${freebsd:+freebsd=$freebsd} \
+  ${linux:+linux=$linux} \
+  ${msys:+msys=$msys} \
+  ${netbsd:+netbsd=$netbsd} \
+  ${openbsd:+openbsd=$openbsd} \
+  ${osx:+osx=$osx} \
+  ${solaris:+solaris=$solaris} \
+  ${win32:+win32=$win32} \
+  "AWK='$AWK'" \
+  "SED='$SED'" \
+  ${HEAD_SYNTAX:+HEAD_SYNTAX=$HEAD_SYNTAX} \
+  ${TAIL_SYNTAX:+TAIL_SYNTAX=$TAIL_SYNTAX} \
+  ${TAIL_NO_DOUBLE_DASH:+TAIL_NO_DOUBLE_DASH=$TAIL_NO_DOUBLE_DASH} \