Allow system GDAL to be used in Gradle builds.

Modified the build.gradle file by adding an entry in the ant-block to
load either the Windows or Linux GDAL properties file based on whether
we are running in Windows or not. The dependencies clause was also
modified by adding a check to see if the "systemGDAL" property was
specified as a property on the command-line during a build. If this was
the case then the GDAL jar on the system will be used during the build,
else the default published one will be used from the maven-central or
jcenter repositories.

Author: wcmatthysen

build.gradle

@@ -20,6 +20,7 @@ targetCompatibility = '1.8'
 
 ant {
     property(file: 'build.properties')
+    property(file: "gdal.${org.gradle.internal.os.OperatingSystem.current().isWindows() ? 'win' : 'unix'}.properties")
 }
 
 ext {
@@ -38,8 +39,12 @@ repositories {
 dependencies {
     compile "org.jogamp.jogl:jogl-all-main:$project.joglVersion"
     compile "org.jogamp.gluegen:gluegen-rt-main:$project.joglVersion"
-
-    implementation "org.gdal:gdal:$project.gdalVersion"
+    
+    if (project.hasProperty('systemGDAL')) {
+        compile files("${ant.properties['gdal.jar.dir']}/gdal.jar")
+    } else {
+        implementation "org.gdal:gdal:$project.gdalVersion"
+    }
 
     compile "org.codehaus.jackson:jackson-core-asl:$project.jacksonVersion"