Make Android.mk threads sensitive.

Make the definition of CERES_NO_THREADS conditional on the presence
of the CERES_HAVE_PTHREADS define.

This allows the user to enable threading in ceres using a combination
of CERES_EXTRA_DEFINES and linker flags which they need to do in
their app.

Change-Id: I43b99fc829001166e2f6dfb0a770768e61470591

jni/Android.mk

diff --git a/jni/Android.mk b/jni/Android.mk
index 45b95b4..cdee79e 100644
--- a/jni/Android.mk
+++ b/jni/Android.mk
@@ -102,10 +102,19 @@
 LOCAL_CFLAGS := $(CERES_EXTRA_DEFINES) \
                 -DCERES_NO_LAPACK \
                 -DCERES_NO_SUITESPARSE \
-                -DCERES_NO_THREADS \
                 -DCERES_NO_CXSPARSE \
                 -DCERES_STD_UNORDERED_MAP
 
+
+# If the user did not enable threads in CERES_EXTRA_DEFINES, then add
+# CERES_NO_THREADS.
+#
+# TODO(sameeragarwal): Update comments here and in the docs to
+# demonstrate how OpenMP can be used by the user.
+ifeq (,$(findstring CERES_HAVE_PTHREAD, $(LOCAL_CFLAGS)))
+  LOCAL_CFLAGS += -DCERES_NO_THREADS
+endif
+
 LOCAL_SRC_FILES := $(CERES_SRC_PATH)/array_utils.cc \
                    $(CERES_SRC_PATH)/blas.cc \
                    $(CERES_SRC_PATH)/block_evaluate_preparer.cc \