build.sh: let caller override CMAKE_PREFIX_PATH

It is not clear exactly what CMAKE_PREFIX_PATH is used for.
In prehistory, it used to point to third-party-install/
and then was changed to third-party-install/lib/cmake
in some conda related commit.
Stick to its current value, but allow the caller to override it
if needed.  Use CONDA_PREFIX directly, since
THIRD_PARTY_INSTALL_PREFIX will be changed in an upcoming commit.

Author: Sven Verdoolaege

build.sh

@@ -22,6 +22,7 @@ WITH_CAFFE2=${WITH_CAFFE2:=OFF}
 WITH_TAPIR=${WITH_TAPIR:=ON}
 
 CUDNN_ROOT_DIR=${CUDNN_ROOT_DIR:=${CONDA_PREFIX}}
+CMAKE_PREFIX_PATH=${CMAKE_PREFIX_PATH:=${CONDA_PREFIX}/lib/cmake}
 
 THIRD_PARTY_INSTALL_PREFIX=${CONDA_PREFIX}
 
@@ -45,7 +46,7 @@ cmake -DWITH_CAFFE2=${WITH_CAFFE2} \
        -DWITH_TAPIR=${WITH_TAPIR} \
        -DPYTHON_EXECUTABLE=${PYTHON} \
        -DCMAKE_BUILD_TYPE=${BUILD_TYPE} \
-       -DCMAKE_PREFIX_PATH=${THIRD_PARTY_INSTALL_PREFIX}/lib/cmake \
+       -DCMAKE_PREFIX_PATH=${CMAKE_PREFIX_PATH} \
        -DTHIRD_PARTY_INSTALL_PREFIX=${THIRD_PARTY_INSTALL_PREFIX} \
        -DCMAKE_INSTALL_PREFIX=${INSTALL_PREFIX} \
        -DCLANG_PREFIX=${CLANG_PREFIX} \