cmake: Attempt to find python interpreter only if BUILD_TESTING is ON

This commit also improves the message reported at configuration time:

```
[...]
-- Build tests: ON
-- Looking for Python >= 2.7 needed for summary tests
-- Looking for Python >= 2.7 needed for summary tests - found (2.7.10)

[...]
```

or

```
[...]
-- Build tests: ON
-- Looking for Python >= 2.7 needed for summary tests
-- Looking for Python >= 2.7 needed for summary tests - not found (skipping summary tests)

[...]
```

Author: jcfr

CMakeLists.txt

@@ -110,16 +110,8 @@ if(CMAKE_Fortran_COMPILER_ID STREQUAL "Compaq")
   endif()
 endif()
 
-# Get Python
-message(STATUS "Looking for Python greater than 2.6 - ${PYTHONINTERP_FOUND}")
-find_package(PythonInterp 2.7) # lapack_testing.py uses features from python 2.7 and greater
-if(PYTHONINTERP_FOUND)
-  message(STATUS "Using Python version ${PYTHON_VERSION_STRING}")
-else()
-  message(STATUS "No suitable Python version found, so skipping summary tests.")
-endif()
-# --------------------------------------------------
 
+# --------------------------------------------------
 set(LAPACK_INSTALL_EXPORT_NAME lapack-targets)
 
 macro(lapack_install_library lib)
@@ -139,6 +131,18 @@ option(BUILD_TESTING "Build tests" ${_is_coverage_build})
 include(CTest)
 message(STATUS "Build tests: ${BUILD_TESTING}")
 
+# lapack_testing.py uses features from python 2.7 and greater
+if(BUILD_TESTING)
+  set(_msg "Looking for Python >= 2.7 needed for summary tests")
+  message(STATUS "${_msg}")
+  find_package(PythonInterp 2.7 QUIET)
+  if(PYTHONINTERP_FOUND)
+    message(STATUS "${_msg} - found (${PYTHON_VERSION_STRING})")
+  else()
+    message(STATUS "${_msg} - not found (skipping summary tests)")
+  endif()
+endif()
+
 # --------------------------------------------------
 # Organize output files.  On Windows this also keeps .dll files next
 # to the .exe files that need them, making tests easy to run.