Always invoke gradle except as root

The custom target used to invoke Gradle from Meson should always
be built, otherwise, the server would not be rebuilt on source changes.

However, when enabling "build_always", gradle is invoked as root on
"sudo ninja install" after "ninja", so it downloads the whole Gradle
world into /root/.gradle.

To avoid the problem, just do not call gradle if the effective user id
is 0.

server/meson.build

@@ -1,9 +1,5 @@
-project('scrcpy-server', 'c')  # not really c, but meson expects something
-
-# does not track dependencies, so meson does not guarantees that server is up to date
-# call "touch server" or "ninja -t clean server/scrcpy-server.jar" before to rebuild
 custom_target('scrcpy-server',
-              build_always: false, # do not enable, otherwise "sudo ninja install" will execute gradle!
+              build_always: true,  # gradle is responsible for tracking source changes
               input: '.',
               output: 'scrcpy-server.jar',
               command: [find_program('./scripts/build-wrapper.sh'), '@INPUT@', '@OUTPUT@', get_option('buildtype')],

server/scripts/build-wrapper.sh

@@ -2,6 +2,16 @@
 # Wrapper script to invoke gradle from meson
 set -e
 
+# Do not execute gradle when ninja is called as root (it would download the
+# whole gradle world in /root/.gradle).
+# This is typically useful for calling "sudo ninja install" after a "ninja
+# install"
+if [[ "$EUID" == 0 ]]
+then
+    echo "(not invoking gradle, since we are root)" >&2
+    exit 0
+fi
+
 PROJECT_ROOT="$1"
 OUTPUT="$2"
 BUILDTYPE="$3"