CMAKE_COMPILER_IS_GNU<LANG>

True if the compiler is GNU.

If the selected <LANG> compiler is the GNU compiler then this is TRUE, if not it is FALSE. Unlike the other per-language variables, this uses the GNU syntax for identifying languages instead of the CMake syntax. Recognized values of the <LANG> suffix are:

CC = C compiler
CXX = C++ compiler
G77 = Fortran compiler

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Licensed under the BSD 3-clause License.
https://cmake.org/cmake/help/v3.6/variable/CMAKE_COMPILER_IS_GNULANG.html