Allow building zlib outside of the source directory.

To build, simply run configure from the source directory by
specifying its path.  That path will be used to find the source
files.  The source directory will not be touched.  All new and
modified files will be made in the current directory.  Discovered
in the process that not all makes understand % or $<, and not all
compilers understand -include or -I-.  This required a larger
Makefile.in with explicit dependencies.

configure

@@ -18,6 +18,18 @@ echo -------------------- >> configure.log
 echo $0 $* >> configure.log
 date >> configure.log
 
+# get source directory
+SRCDIR=`dirname $0`
+if test $SRCDIR = "."; then
+    ZINC=""
+    ZINCOUT="-I."
+    SRCDIR=""
+else
+    ZINC='-include zconf.h'
+    ZINCOUT='-I. -I$(SRCDIR)'
+    SRCDIR="$SRCDIR/"
+fi
+
 # set command prefix for cross-compilation
 if [ -n "${CHOST}" ]; then
     uname="`echo "${CHOST}" | sed -e 's/^[^-]*-\([^-]*\)$/\1/' -e 's/^[^-]*-[^-]*-\([^-]*\)$/\1/' -e 's/^[^-]*-[^-]*-\([^-]*\)-.*$/\1/'`"
@@ -28,10 +40,10 @@ fi
 STATICLIB=libz.a
 
 # extract zlib version numbers from zlib.h
-VER=`sed -n -e '/VERSION "/s/.*"\(.*\)".*/\1/p' < zlib.h`
-VER3=`sed -n -e '/VERSION "/s/.*"\([0-9]*\\.[0-9]*\\.[0-9]*\).*/\1/p' < zlib.h`
-VER2=`sed -n -e '/VERSION "/s/.*"\([0-9]*\\.[0-9]*\)\\..*/\1/p' < zlib.h`
-VER1=`sed -n -e '/VERSION "/s/.*"\([0-9]*\)\\..*/\1/p' < zlib.h`
+VER=`sed -n -e '/VERSION "/s/.*"\(.*\)".*/\1/p' < ${SRCDIR}zlib.h`
+VER3=`sed -n -e '/VERSION "/s/.*"\([0-9]*\\.[0-9]*\\.[0-9]*\).*/\1/p' < ${SRCDIR}zlib.h`
+VER2=`sed -n -e '/VERSION "/s/.*"\([0-9]*\\.[0-9]*\)\\..*/\1/p' < ${SRCDIR}zlib.h`
+VER1=`sed -n -e '/VERSION "/s/.*"\([0-9]*\)\\..*/\1/p' < ${SRCDIR}zlib.h`
 
 # establish commands for library building
 if "${CROSS_PREFIX}ar" --version >/dev/null 2>/dev/null || test $? -lt 126; then
@@ -184,9 +196,9 @@ if test "$gcc" -eq 1 && ($cc -c $test.c) >> configure.log 2>&1; then
   fi
   case "$uname" in
   Linux* | linux* | GNU | GNU/* | solaris*)
-        LDSHARED=${LDSHARED-"$cc -shared -Wl,-soname,libz.so.1,--version-script,zlib.map"} ;;
+        LDSHARED=${LDSHARED-"$cc -shared -Wl,-soname,libz.so.1,--version-script,${SRCDIR}zlib.map"} ;;
   *BSD | *bsd* | DragonFly)
-        LDSHARED=${LDSHARED-"$cc -shared -Wl,-soname,libz.so.1,--version-script,zlib.map"}
+        LDSHARED=${LDSHARED-"$cc -shared -Wl,-soname,libz.so.1,--version-script,${SRCDIR}zlib.map"}
         LDCONFIG="ldconfig -m" ;;
   CYGWIN* | Cygwin* | cygwin* | OS/2*)
         EXE='.exe' ;;
@@ -287,6 +299,9 @@ else
                  esac
              fi
          fi
+         if test -n "$ZINC"; then
+             ZINC='-I- -I. -I$(SRCDIR)'
+         fi
          ;;
   SunOS\ 4*) SFLAGS=${CFLAGS-"-O2 -PIC"}
              CFLAGS=${CFLAGS-"-O2"}
@@ -470,7 +485,7 @@ else
 fi
 
 # copy clean zconf.h for subsequent edits
-cp -p zconf.h.in zconf.h
+cp -p ${SRCDIR}zconf.h.in zconf.h
 
 echo >> configure.log
 
@@ -764,6 +779,7 @@ echo STATICLIB = $STATICLIB >> configure.log
 echo TEST = $TEST >> configure.log
 echo VER = $VER >> configure.log
 echo Z_U4 = $Z_U4 >> configure.log
+echo SRCDIR = $SRCDIR >> configure.log
 echo exec_prefix = $exec_prefix >> configure.log
 echo includedir = $includedir >> configure.log
 echo libdir = $libdir >> configure.log
@@ -773,7 +789,7 @@ echo sharedlibdir = $sharedlibdir >> configure.log
 echo uname = $uname >> configure.log
 
 # udpate Makefile with the configure results
-sed < Makefile.in "
+sed < ${SRCDIR}Makefile.in "
 /^CC *=/s#=.*#=$CC#
 /^CFLAGS *=/s#=.*#=$CFLAGS#
 /^SFLAGS *=/s#=.*#=$SFLAGS#
@@ -790,6 +806,9 @@ sed < Makefile.in "
 /^LDCONFIG *=/s#=.*#=$LDCONFIG#
 /^LDSHAREDLIBC *=/s#=.*#=$LDSHAREDLIBC#
 /^EXE *=/s#=.*#=$EXE#
+/^SRCDIR *=/s#=.*#=$SRCDIR#
+/^ZINC *=/s#=.*#=$ZINC#
+/^ZINCOUT *=/s#=.*#=$ZINCOUT#
 /^prefix *=/s#=.*#=$prefix#
 /^exec_prefix *=/s#=.*#=$exec_prefix#
 /^libdir *=/s#=.*#=$libdir#
@@ -803,7 +822,7 @@ sed < Makefile.in "
 " > Makefile
 
 # create zlib.pc with the configure results
-sed < zlib.pc.in "
+sed < ${SRCDIR}zlib.pc.in "
 /^CC *=/s#=.*#=$CC#
 /^CFLAGS *=/s#=.*#=$CFLAGS#
 /^CPP *=/s#=.*#=$CPP#