CMAKE_CXX_COMPILER_VERSION is pointing to the old GCC version - c++

Question

I have upgraded my GCC using:
$ sudo add-apt-repository ppa:ubuntu-toolchain-r/test
$ sudo apt-get update
$ sudo apt-get install gcc-8 g++-8
$ sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-8 70 --slave /usr/bin/g++ g++ /usr/bin/g++-8
Running any of these commands:
$ gcc --version
$ g++ --version
$ c++ --version
$ /usr/bin/gcc --version
$ /usr/bin/g++ --version
$ /usr/bin/c++ --version
would show (Ubuntu 8.1.0-5ubuntu1~16.04) 8.1.0 confirming that version 8.1 has been installed.
When running ./configure on cmake-3.12.1 I downloaded from its website I get:
-- The C compiler identification is GNU 8.1.0
-- The CXX compiler identification is GNU 8.1.0
However when trying to make my actual project:
CMake Error at CMakeLists.txt:24 (message):
GCC version must be at least 7.1! 5.4.0
This is my CMakeLists.txt:
if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
# require at least gcc 7.1
if (CMAKE_CXX_COMPILER_VERSION VERSION_LESS 7.1)
message(FATAL_ERROR "GCC version must be at least 7.1! " ${CMAKE_CXX_COMPILER_VERSION})
endif()
endif()

Answer 1

As Shawn, Tsyvarev and hellow have mentioned in the comments, this problem is caused by CMake cache file which was located inside /build/. Deleting the file solved the issue.