Do not search for PCL components that have been found already

[taketwo]

Fixes #2427.

[jasjuang]

This fixed the minimal example I mentioned in #2427 but it doesn't completely fix it. Here is another minimal example that induces the same error

CMakeLists.txt

cmake_minimum_required(VERSION 3.11)

add_subdirectory(sample2)
add_subdirectory(sample3)

sample2/CMakeLists.txt

cmake_minimum_required(VERSION 3.11)

project(sample2)

find_package( PCL REQUIRED )

add_library(${PROJECT_NAME} SHARED Sample2.cpp Sample2.h)

target_link_libraries(${PROJECT_NAME} ${PCL_LIBRARIES})

sample2/Sample2.h

class Sample2{
 public:
  Sample2();
  ~Sample2();
};

sample2/Sample2.cpp

#include "Sample2.h"

Sample2::Sample2() {}

Sample2::~Sample2() {}

sample3/CMakeLists.txt

cmake_minimum_required(VERSION 3.11)

project(sample3)

find_package( PCL REQUIRED )

add_library(${PROJECT_NAME} SHARED Sample3.cpp Sample3.h)

target_link_libraries(${PROJECT_NAME} ${PCL_LIBRARIES})

sample3/Sample3.h

class Sample3{
 public:
  Sample3();
  ~Sample3();
};

sample3/Sample3.cpp

#include "Sample3.h"

Sample3::Sample3() {}

Sample3::~Sample3() {}

You can see that sample2 and sample3 are irrelevant to each other yet it induces the same cmake error. Gist coming shortly.

[taketwo]

No need for the Gist, I see the problem.

[jasjuang]

@taketwo Here is the gist https://gist.github.com/jasjuang/41a644f5b6ac1c4a092cde404d17e2ca

[taketwo]

OK, I've changed how the "has been found already" test is carried out. Instead of relying on PCL_xxx_FOUND variable (which is not visible across sibling subdirectories) we now check existence of pcl_xxx target.

[jasjuang]

Thanks. I tested and it works. Please merge. However, I ran into another separate problem and I will file a separate issue for it.