String comparisons in CMake failing

[simonkampe]

When building abseil-cpp with CMake > 3.1 the string comparisons will fail if there exists a variable with the same name as the actual string.

Discovered when compiling abseil as part of a larger project, we had an option called shared in another subrepos CMake-files, and this collided with the check in AbseilHelpers.cpp causing it to hit this else() clause:

abseil-cpp/CMake/AbseilHelpers.cmake

Line 180 in 9927a09

message(FATAL_ERROR "Invalid build type: ${_build_type}")

Some debugging suggests this can be solved by changing the comparisons from e.g.

elseif(${_build_type} STREQUAL "static" OR ${_build_type} STREQUAL "shared")

to

elseif(_build_type STREQUAL "static" OR _build_type STREQUAL "shared")

or by using

cmake_policy(SET CMP0054 OLD)

for CMake > 3.1

Somewhere in our CMake-files we set the CMake policy to:

cmake_policy(VERSION ${CMAKE_MAJOR_VERSION}.${CMAKE_MINOR_VERSION})

which should result in it using the NEW behaviour that causes the STREQUALs to fail since it now dereferences everything.