This should insure that both the `PythonLibs` and `PythonInterp` points to
the same python version.
Without this, the Python library and interpreter might not match version.
For example, if both Python 2.7 and 3.4 is installed, `PythonLibs` will
find /usr/lib/x86_64-linux-gnu/libpython3.4m.so while
`PythonInterp` will find /usr/bin/python2.7 (at least on Ubuntu 14.04).
When `PythonLibs` and `PythonInterp` don't point to the same Python version,
the examples will all fail:
```bash
$ cmake ..
-- The C compiler identification is GNU 4.8.4
-- The CXX compiler identification is GNU 4.8.4
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Setting build type to 'MinSizeRel' as none was specified.
-- Found PythonLibs: /usr/lib/x86_64-linux-gnu/libpython3.4m.so (found suitable version "3.4.3", minimum required is "2.7")
-- Found PythonInterp: /usr/bin/python2.7 (found suitable version "2.7.6", minimum required is "2.7")
-- Performing Test HAS_LTO_FLAG
-- Performing Test HAS_LTO_FLAG - Success
-- Configuring done
-- Generating done
-- Build files have been written to: /home/nbigaouette/pybind11/build
$ make test
Running tests...
Test project /home/nbigaouette/pybind11/build
Start 1: example1
1/12 Test #1: example1 .........................***Failed 0.02 sec
Start 2: example2
2/12 Test #2: example2 .........................***Failed 0.03 sec
Start 3: example3
3/12 Test #3: example3 .........................***Failed 0.02 sec
Start 4: example4
4/12 Test #4: example4 .........................***Failed 0.02 sec
Start 5: example5
5/12 Test #5: example5 .........................***Failed 0.02 sec
Start 6: example6
6/12 Test #6: example6 .........................***Failed 0.02 sec
Start 7: example7
7/12 Test #7: example7 .........................***Failed 0.02 sec
Start 8: example8
8/12 Test #8: example8 .........................***Failed 0.02 sec
Start 9: example9
9/12 Test #9: example9 .........................***Failed 0.02 sec
Start 10: example10
10/12 Test #10: example10 ........................***Failed 0.02 sec
Start 11: example11
11/12 Test #11: example11 ........................***Failed 0.03 sec
Start 12: example12
12/12 Test #12: example12 ........................***Failed 0.02 sec
0% tests passed, 12 tests failed out of 12
Total Test time (real) = 0.25 sec
The following tests FAILED:
1 - example1 (Failed)
2 - example2 (Failed)
3 - example3 (Failed)
4 - example4 (Failed)
5 - example5 (Failed)
6 - example6 (Failed)
7 - example7 (Failed)
8 - example8 (Failed)
9 - example9 (Failed)
10 - example10 (Failed)
11 - example11 (Failed)
12 - example12 (Failed)
Errors while running CTest
make: *** [test] Error 8
```
By adding the `EXACT` version to the `find_package()` calls, the version
discrepancy is at least caught at the cmake call:
```bash
$ cmake ..
-- The C compiler identification is GNU 4.8.4
-- The CXX compiler identification is GNU 4.8.4
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Setting build type to 'MinSizeRel' as none was specified.
CMake Error at /usr/share/cmake-2.8/Modules/FindPackageHandleStandardArgs.cmake:108 (message):
Could NOT find PythonLibs: Found unsuitable version "3.4.3", but required
is exact version "2.7" (found /usr/lib/x86_64-linux-gnu/libpython3.4m.so)
Call Stack (most recent call first):
/usr/share/cmake-2.8/Modules/FindPackageHandleStandardArgs.cmake:313 (_FPHSA_FAILURE_MESSAGE)
/usr/share/cmake-2.8/Modules/FindPythonLibs.cmake:208 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
CMakeLists.txt:27 (find_package)
-- Configuring incomplete, errors occurred!
See also "/home/nbigaouette/pybind11/build/CMakeFiles/CMakeOutput.log".
```
