perf jvmti: improve error message in Makefile

This patch improves the error message given by jvmti Makefile when the
alternatives command cannot be found. It now suggests the user locates
the root of their Java installation and pass it with JDIR=

Signed-off-by: Stephane Eranian <eranian@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1456378056-18812-1-git-send-email-eranian@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/jvmti/Makefile

@@ -35,12 +35,21 @@ SOLIBEXT=so
 
 # The following works at least on fedora 23, you may need the next
 # line for other distros.
-ifeq (,$(wildcard /usr/sbin/update-java-alternatives))
-JDIR=$(shell alternatives --display java | tail -1 | cut -d' ' -f 5 | sed 's%/jre/bin/java.%%g')
-else
+ifneq (,$(wildcard /usr/sbin/update-java-alternatives))
 JDIR=$(shell /usr/sbin/update-java-alternatives -l | head -1 | cut -d ' ' -f 3)
+else
+  ifneq (,$(wildcard /usr/sbin/alternatives))
+    JDIR=$(shell alternatives --display java | tail -1 | cut -d' ' -f 5 | sed 's%/jre/bin/java.%%g')
+  endif
 endif
-
+ifndef JDIR
+$(error Could not find alternatives command, you need to set JDIR= to point to the root of your Java directory)
+else
+  ifeq (,$(wildcard $(JDIR)/include/jvmti.h))
+  $(error the openjdk development package appears to me missing, install and try again)
+  endif
+endif
+$(info Using Java from $(JDIR))
 # -lrt required in 32-bit mode for clock_gettime()
 LIBS=-lelf -lrt
 INCDIR=-I $(JDIR)/include -I $(JDIR)/include/linux