diff -rupN librttopo/configure.ac librttopo-new/configure.ac
--- librttopo/configure.ac	2019-07-27 08:55:02.000000000 +0200
+++ librttopo-new/configure.ac	2021-02-27 10:13:26.397946908 +0100
@@ -84,36 +84,10 @@ AC_CONFIG_FILES([Makefile \
 		headers/librttopo_geom.h \
 		rttopo.pc])
 
-#-----------------------------------------------------------------------
-#   --with-geosconfig
-#
-AC_ARG_WITH([geosconfig],
-    [AS_HELP_STRING([--with-geosconfig=FILE], [specify an alternative geos-config file])],
-	[GEOSCONFIG="$withval"], [GEOSCONFIG=""])
-if test "x$GEOSCONFIG" = "x"; then
-    # GEOSCONFIG was not specified, so search within the current path
-    AC_PATH_PROG([GEOSCONFIG], [geos-config])
-    # If we couldn't find geos-config, display an error
-    if test "x$GEOSCONFIG" = "x"; then
-            AC_MSG_ERROR([could not find geos-config within the current path. You may need to try re-running configure with a --with-geosconfig parameter.])
-    fi
-else
-    # GEOSCONFIG was specified; display a message to the user
-    if test "x$GEOSCONFIG" = "xyes"; then
-            AC_MSG_ERROR([you must specify a parameter to --with-geosconfig, e.g. --with-geosconfig=/path/to/geos-config])
-    else
-            if test -f $GEOSCONFIG; then
-                    AC_MSG_RESULT([Using user-specified geos-config file: $GEOSCONFIG])
-            else
-                    AC_MSG_ERROR([the user-specified geos-config file $GEOSCONFIG does not exist])
-            fi
-    fi
-fi
-# Extract the linker and include flags
-GEOS_LDFLAGS=`$GEOSCONFIG --ldflags`
-GEOS_CFLAGS=-I`$GEOSCONFIG --includes`
-AC_SUBST([GEOS_LDFLAGS])
-AC_SUBST([GEOS_CFLAGS])
+PKG_CHECK_MODULES([GEOS], [geos], , AC_MSG_ERROR(['geos' is required but it doesn't seem to be installed on this system.]))
+GEOS_LDFLAGS="$GEOS_LIBS"
+AC_SUBST(GEOS_CFLAGS)
+AC_SUBST(GEOS_LDFLAGS)
 # Ensure that we can parse geos_c.h
 CPPFLAGS_SAVE="$CPPFLAGS"
 CPPFLAGS="$GEOS_CFLAGS"
@@ -125,8 +99,8 @@ LIBS="$GEOS_LDFLAGS"
 AC_SEARCH_LIBS(GEOSContext_setErrorMessageHandler_r,geos_c,,AC_MSG_ERROR([could not find libgeos_c (or obsolete 'libgeos_c' < v.3.5.0 found) - you may need to specify the directory of a geos-config file using --with-geosconfig]))
 LIBS="$LIBS_SAVE"
 LIBS="$LIBS $GEOS_LDFLAGS -lgeos_c"
-GEOS_MAJOR_VERSION=`$GEOSCONFIG --version | cut -d. -f1 | sed 's/[[^0-9]]//g'`
-GEOS_MINOR_VERSION=`$GEOSCONFIG --version | cut -d. -f2 | sed 's/[[^0-9]]//g'`
+GEOS_MAJOR_VERSION=`${PKGCONFIG:-pkg-config} --modversion geos | cut -d. -f1 | sed 's/[[^0-9]]//g'`
+GEOS_MINOR_VERSION=`${PKGCONFIG:-pkg-config} --modversion geos | cut -d. -f2 | sed 's/[[^0-9]]//g'`
 RTGEOM_GEOS_VERSION="$GEOS_MAJOR_VERSION$GEOS_MINOR_VERSION"
 AC_DEFINE_UNQUOTED([RTGEOM_GEOS_VERSION], [$RTGEOM_GEOS_VERSION], [GEOS library version])
 AC_SUBST([RTGEOM_GEOS_VERSION])