refactor zlib's spkg-configure, start on a macro for zlib.pc

this is WIP.

build/pkgs/zlib/spkg-configure.m4

@@ -1,13 +1,38 @@
+AC_DEFUN([SAGE_ZLIB_GEN_PC],[dnl generate zlib.pc
+  AX_ABSOLUTE_HEADER([zlib.h])
+  AS_IF([test x$gl_cv_absolute_zlib_h = x], [
+     AC_MSG_ERROR(m4_normalize([
+       failed to find absolute path to zlib.h despite it being reported found
+     ]))
+  ])
+  AC_SUBST(SAGE_ZLIB_INCLUDE, [`AS_DIRNAME($gl_cv_absolute_zlib_h)`])
+  dnl now the same for the full path to libz...
+  dnl ...
+])
+
 SAGE_SPKG_CONFIGURE([zlib], [
-    AC_CHECK_LIB([z], [inflateEnd], [zlib_cv_libz=yes], [zlib_cv_libz=no])
-    AC_CHECK_HEADER([zlib.h], [zlib_cv_zlib_h=yes], [zlib_cv_zlib_h=no])
-    if test "$zlib_cv_libz" = "yes" && test "$zlib_cv_zlib_h" = "yes"; then
-        PKG_CHECK_MODULES([LIBPNG], [libpng >= 1.2], [], [
-            dnl inflateValidate is needed for Sage's libpng, newer than 1.2; this ensures
-            dnl we have the minimum required for building zlib version
-            AC_CHECK_LIB([z], [inflateValidate], [], [sage_spkg_install_zlib=yes])
-        ])
-    else
-        sage_spkg_install_zlib=yes
-    fi
+    PKG_CHECK_MODULES([ZLIBANDLIBPNG], [zlib libpng >= 1.2], [
+      AC_MSG_NOTICE([Use zlib and libpng from the system, pc files supplied.])
+      ], [
+      PKG_CHECK_MODULES([ZLIB], [zlib >= 1.2.9], [
+        dnl inflateValidate is needed for Sage's libpng, newer than 1.2; this ensures
+        dnl we have the minimum required for building zlib version
+        AC_MSG_NOTICE([Use zlib the system, pc file supplied.])
+        ], [dnl no zlib.pc file. try to see if this zlib is still good
+        AC_CHECK_HEADER([zlib.h], [
+           AC_CHECK_LIB([z], [inflateEnd], [
+              PKG_CHECK_MODULES([LIBPNG], [libpng >= 1.2], [
+                dnl hope we found the zlib of system's libpng
+                SAGE_ZLIB_GEN_PC
+                ], [
+                dnl inflateValidate is needed for Sage's libpng (cf. above)
+                AC_CHECK_LIB([z], [inflateValidate], [
+                   dnl zlib is good enough to build Sage's libpng
+                   SAGE_ZLIB_GEN_PC
+                   ], [sage_spkg_install_zlib=yes])
+                ])
+           ], [sage_spkg_install_zlib=yes])
+        ], [sage_spkg_install_zlib=yes])
+      ])
+    ])
 ])