avoid calling "find" on an empty CONDA_PREFIX

In the prehistory, the CONDA_PREFIX variable was introduced in build.sh to
avoid compiling and installing third-party dependences already installed
as conda packages.  However, the check for the libraries being present
is inaccurate as it uses 'find "$CONDA_PREFIX" lib*.so' where
CONDA_PREFIX may be (and in non-conda builds is) empty.  In the case of
empty path, "find" searches for the given files in recursively starting
from the current directory.  When called after a previous successful
build, "find" finds the libraries in their _build_ directories and skips
the following compilation passes, including checks for changes,
entirely.  It also does not install the libraries if they were removed.

Check that CONDA_PREFIX is not empty before using find to detect
pre-installed libraries.

Author: ftynse

build.sh

@@ -467,7 +467,7 @@ function install_halide() {
 }
 
 if ! test -z $gflags || ! test -z $all; then
-  if [[ $(find $CONDA_PREFIX -name libgflags.so) ]]; then
+  if [[ ! -z "$CONDA_PREFIX" && $(find $CONDA_PREFIX -name libgflags.so) ]]; then
       echo "gflags found"
   else
       echo "no files found"
@@ -476,7 +476,7 @@ if ! test -z $gflags || ! test -z $all; then
 fi
 
 if ! test -z $glog || ! test -z $all; then
-    if [[ $(find $CONDA_PREFIX -name libglog.so) ]]; then
+    if [[ ! -z "$CONDA_PREFIX" && $(find $CONDA_PREFIX -name libglog.so) ]]; then
         echo "glog found"
     else
         echo "no files found"
@@ -494,7 +494,7 @@ fi
 
 if ! test -z $caffe2 || ! test -z $all ; then
     if [ "$WITH_CAFFE2" == "ON" ]; then
-        if [[ $(find $CONDA_PREFIX -name libcaffe2_gpu.so) ]]; then
+        if [[ ! -z "$CONDA_PREFIX" && $(find $CONDA_PREFIX -name libcaffe2_gpu.so) ]]; then
             echo "caffe2 found"
         else
             echo "no files found"
@@ -504,7 +504,7 @@ if ! test -z $caffe2 || ! test -z $all ; then
 fi
 
 if ! test -z $isl || ! test -z $all; then
-    if [[ $(find $CONDA_PREFIX -name libisl.so) ]]; then
+    if [[ ! -z "$CONDA_PREFIX" && $(find $CONDA_PREFIX -name libisl.so) ]]; then
         echo "isl found"
     else
         echo "no files found"
@@ -517,7 +517,7 @@ if ! test -z $dlpack || ! test -z $all; then
 fi
 
 if ! test -z $halide || ! test -z $all; then
-    if [[ $(find $CONDA_PREFIX -name libHalide.so) ]]; then
+    if [[ ! -z "$CONDA_PREFIX" && $(find $CONDA_PREFIX -name libHalide.so) ]]; then
         echo "Halide found"
     else
         echo "no files found"