On my Ubuntu 18.04.5 system (but not other Linux systems) I'm seeing make always rebuilding libjvm.so and so taking a long time to do the null build.

 $ /usr/bin/make
Building target 'default (jdk-image test-image-jdk-jtreg-native)' in configuration 'linux-x86_64-server-release'
Updating support/modules_libs/java.base/server/libjvm.so due to 9 file(s)

Configured using:

 bash ./configure --with-toolchain-type=gcc --with-extra-cflags=-Wno-deprecated-declarations --disable-warnings-as-errors --with-cacerts-file=/etc/ssl/certs/java/cacerts --enable-unlimited-crypto --with-native-debug-symbols=none --with-default-make-target=jdk-image test-image-jdk-jtreg-native --with-jtreg=/home/martin/jtreg-binaries/5.1-b01 --with-jmh=/home/martin/jmh-binaries/jmh-1.21 --with-jvm-features=-shenandoahgc --with-boot-jdk=/home/martin/jdk/jdk15

Not sure, but I think this is a recent build regression for tip of openjdk/jdk