Allow more typical --prefix= syntax (#744)

Co-authored-by: John Kerl <kerl.john.r@gmail.com>

configure

@@ -2,21 +2,34 @@
 
 usage() {
   echo "Usage: $0 [--prefix {prefix directory}], e.g. /usr/local" 2>&1
+  echo "    or $0 [--prefix={prefix directory}], e.g. /usr/local" 2>&1
   exit 1
 }
 
 prefix="/usr/local"
+
 if [ $# -eq 0 ]; then
   echo "Using $prefix"
-elif [ $# -ne 2 ]; then
+elif [ $# -gt 2 ]; then
   usage
 else
-  if [ "$1" != "--prefix" ]; then
-    usage
-  fi
+  while [ $# -gt 0 ]; do
+    case "$1" in
+      -p=* | --prefix=*)
+        prefix="${1#*=}"
+        ;;
+      -p | --prefix)
+        prefix="$2"
+        shift
+        ;;
+      *)
+        printf "Error: unknown option: $1\n"
+        usage
+    esac
+    shift
+  done
   # Test for read access, not write access, since people can ./configure, make,
   # and make check outside of sudo and then make install inside sudo.
-  prefix="$2"
   if [ ! -d "$prefix" ]; then
     echo "$0: could not find prefix directory \"$prefix\""
     exit 1
@@ -24,4 +37,4 @@ else
 fi
 
 sed -I .prefix-backup 's@^PREFIX.*@PREFIX='$prefix'@' Makefile
-sed -I .prefix-backup 's@^PREFIX.*@PREFIX='$prefix'@' man/Makefile
+sed -I .prefix-backup 's@^PREFIX.*@PREFIX='$prefix'@' man/Makefile