CMake: Verify pyAMReX_..._src paths exit (#182)

Give a clean error message if a `pyAMReX_thirdparty_src` directory
does not exist.

Author: ax3l

cmake/dependencies/AMReX.cmake

@@ -4,6 +4,9 @@ macro(find_amrex)
     elseif(pyAMReX_amrex_src)
         message(STATUS "Compiling local AMReX ...")
         message(STATUS "AMReX source path: ${pyAMReX_amrex_src}")
+        if(NOT IS_DIRECTORY ${pyAMReX_amrex_src})
+            message(FATAL_ERROR "Specified directory pyAMReX_amrex_src='${pyAMReX_amrex_src}' does not exist!")
+        endif()
     elseif(pyAMReX_amrex_internal)
         message(STATUS "Downloading AMReX ...")
         message(STATUS "AMReX repository: ${pyAMReX_amrex_repo} (${pyAMReX_amrex_branch})")

cmake/dependencies/pybind11.cmake

@@ -4,6 +4,9 @@ function(find_pybind11)
     elseif(pyAMReX_pybind11_src)
         message(STATUS "Compiling local pybind11 ...")
         message(STATUS "pybind11 source path: ${pyAMReX_pybind11_src}")
+        if(NOT IS_DIRECTORY ${pyAMReX_pybind11_src})
+            message(FATAL_ERROR "Specified directory pyAMReX_pybind11_src='${pyAMReX_pybind11_src}' does not exist!")
+        endif()
     elseif(pyAMReX_pybind11_internal)
         message(STATUS "Downloading pybind11 ...")
         message(STATUS "pybind11 repository: ${pyAMReX_pybind11_repo} (${pyAMReX_pybind11_branch})")