The latest version (5.1.1) of the gnu compiler returns just its major version ("5") when given the "-dumpversion" argument. Gnu compiler developers don't consider this to be an error. A workaround for this case has been made.

cmake/OpenCVDetectCXXCompiler.cmake

@@ -76,17 +76,27 @@ elseif(CMAKE_COMPILER_IS_GNUCXX)
                 OUTPUT_STRIP_TRAILING_WHITESPACE)
 
   # Typical output in CMAKE_OPENCV_GCC_VERSION_FULL: "c+//0 (whatever) 4.2.3 (...)"
-  # Look for the version number
+  # Look for the version number, major.minor.build
   string(REGEX MATCH "[0-9]+\\.[0-9]+\\.[0-9]+" CMAKE_GCC_REGEX_VERSION "${CMAKE_OPENCV_GCC_VERSION_FULL}")
-  if(NOT CMAKE_GCC_REGEX_VERSION)
+  if(NOT CMAKE_GCC_REGEX_VERSION)#major.minor
     string(REGEX MATCH "[0-9]+\\.[0-9]+" CMAKE_GCC_REGEX_VERSION "${CMAKE_OPENCV_GCC_VERSION_FULL}")
   endif()
 
-  # Split the three parts:
-  string(REGEX MATCHALL "[0-9]+" CMAKE_OPENCV_GCC_VERSIONS "${CMAKE_GCC_REGEX_VERSION}")
-
-  list(GET CMAKE_OPENCV_GCC_VERSIONS 0 CMAKE_OPENCV_GCC_VERSION_MAJOR)
-  list(GET CMAKE_OPENCV_GCC_VERSIONS 1 CMAKE_OPENCV_GCC_VERSION_MINOR)
+  if(CMAKE_GCC_REGEX_VERSION)
+    # Split the parts:
+    string(REGEX MATCHALL "[0-9]+" CMAKE_OPENCV_GCC_VERSIONS "${CMAKE_GCC_REGEX_VERSION}")
+
+    list(GET CMAKE_OPENCV_GCC_VERSIONS 0 CMAKE_OPENCV_GCC_VERSION_MAJOR)
+    list(GET CMAKE_OPENCV_GCC_VERSIONS 1 CMAKE_OPENCV_GCC_VERSION_MINOR)
+  else()#compiler returned just the major version number
+    string(REGEX MATCH "[0-9]+" CMAKE_GCC_REGEX_VERSION "${CMAKE_OPENCV_GCC_VERSION_FULL}")
+    if(NOT CMAKE_GCC_REGEX_VERSION)#compiler did not return anything reasonable
+      set(CMAKE_GCC_REGEX_VERSION "0")
+      message(WARNING "GCC version not detected!")
+    endif()
+    set(CMAKE_OPENCV_GCC_VERSION_MAJOR ${CMAKE_GCC_REGEX_VERSION})
+    set(CMAKE_OPENCV_GCC_VERSION_MINOR 0)
+  endif()
 
   set(CMAKE_OPENCV_GCC_VERSION ${CMAKE_OPENCV_GCC_VERSION_MAJOR}${CMAKE_OPENCV_GCC_VERSION_MINOR})
   math(EXPR CMAKE_OPENCV_GCC_VERSION_NUM "${CMAKE_OPENCV_GCC_VERSION_MAJOR}*100 + ${CMAKE_OPENCV_GCC_VERSION_MINOR}")