How to build it from sources?

[bvn13]

OS: Fedora 29
JAVA: 1.8 | 11 - both

issue with JAVA 1.8:

JAVA_HOME=/usr/java/latest ./gradlew build                                                                                         
> Task :compileJava FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':compileJava'.
> Could not target platform: 'Java SE 11' using tool chain: 'JDK 8 (1.8)'.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 0s
1 actionable task: 1 executed

issue with JAVA 11:

JAVA_HOME=/usr/java/jdk-11.0.2 ./gradlew build --info                                                                              
Initialized native services in: /home/bvn13/.gradle/native
Found daemon DaemonInfo{pid=5758, address=[b82b6024-c8ec-4436-a205-444355f67431 port:38887, addresses:[/0:0:0:0:0:0:0:1, /127.0.0.1]], state=Idle, lastBusy=1553359295367, context=DefaultDaemonContext[uid=d2ed7581-de73-4df7-9ff6-933d2268b518,javaHome=/usr/java/jdk1.8.0_201-amd64,daemonRegistryDir=/home/bvn13/.gradle/daemon,pid=5758,idleTimeout=10800000,daemonOpts=-XX:+HeapDumpOnOutOfMemoryError,-Xmx1024m,-Dfile.encoding=UTF-8,-Duser.country=US,-Duser.language=en,-Duser.variant]} however its context does not match the desired criteria.
Java home is different.
Wanted: DefaultDaemonContext[uid=null,javaHome=/usr/java/jdk-11.0.2,daemonRegistryDir=/home/bvn13/.gradle/daemon,pid=16386,idleTimeout=null,daemonOpts=-XX:+HeapDumpOnOutOfMemoryError,-Xmx1024m,-Dfile.encoding=UTF-8,-Duser.country=US,-Duser.language=en,-Duser.variant]
Actual: DefaultDaemonContext[uid=d2ed7581-de73-4df7-9ff6-933d2268b518,javaHome=/usr/java/jdk1.8.0_201-amd64,daemonRegistryDir=/home/bvn13/.gradle/daemon,pid=5758,idleTimeout=10800000,daemonOpts=-XX:+HeapDumpOnOutOfMemoryError,-Xmx1024m,-Dfile.encoding=UTF-8,-Duser.country=US,-Duser.language=en,-Duser.variant]

  Looking for a different daemon...
The client will now receive all logging from the daemon (pid: 6001). The daemon log file: /home/bvn13/.gradle/daemon/4.7/daemon-6001.out.log
Starting 5th build in daemon [uptime: 2 hrs 12 mins 59.528 secs, performance: 92%]
Using 4 worker leases.

FAILURE: Build failed with an exception.

* What went wrong:
Could not create service of type ScriptPluginFactory using BuildScopeServices.createScriptPluginFactory().
> Could not create service of type PluginResolutionStrategyInternal using BuildScopeServices.createPluginResolutionStrategy().

* Try:
Run with --stacktrace option to get the stack trace. Run with --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 0s

[fba-rio]

I could not build it on Ubuntu 18.10 either.
OpenJDK 11 + OpenJFX.

$ ./gradlew build --info
Initialized native services in: /home/fba/.gradle/native
The client will now receive all logging from the daemon (pid: 20924). The daemon log file: /home/fba/.gradle/daemon/4.7/daemon-20924.out.log
Starting 4th build in daemon [uptime: 1 hrs 56 mins 3.712 secs, performance: 96%]
Using 8 worker leases.
Invalidating in-memory cache of /tmp/jmonkeybuilder/.gradle/4.7/fileHashes/fileHashes.bin

FAILURE: Build failed with an exception.

* What went wrong:
Could not create service of type ScriptPluginFactory using BuildScopeServices.createScriptPluginFactory().
> Could not create service of type PluginResolutionStrategyInternal using BuildScopeServices.createPluginResolutionStrategy().

* Try:
Run with --stacktrace option to get the stack trace. Run with --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 1s

[pateman]

I can't seem to build it either.
Windows 10 Pro, Oracle Java 11.0.1

$ ./gradlew.bat build -Dorg.gradle.java.home="c:/program files/java/jdk-11.0.1"

FAILURE: Build failed with an exception.

* What went wrong:
Could not create service of type ScriptPluginFactory using BuildScopeServices.createScriptPluginFactory().
> Could not create service of type PluginResolutionStrategyInternal using BuildScopeServices.createPluginResolutionStrategy().

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 1s

[JavaSaBr]

for java 8 you need to build from tag 1.7.2