Full path to ASM sometimes not correctly determined

As an example, consider the following input:

# CMakeLists.txt
cmake_minimum_required(VERSION 3.8)
project(test LANGUAGES Fortran)
enable_language(ASM)
add_executable(main main.f90)

# main.f90
end

Configuring with -DCMAKE_C_COMPILER=gcc results in

-- The Fortran compiler identification is GNU 8.2.1
-- Check for working Fortran compiler: /bin/f95
-- Check for working Fortran compiler: /bin/f95  -- works
-- Detecting Fortran compiler ABI info
-- Detecting Fortran compiler ABI info - done
-- Checking whether /bin/f95 supports Fortran 90
-- Checking whether /bin/f95 supports Fortran 90 -- yes
-- The ASM compiler identification is GNU
-- Found assembler: gcc
CMake Error at CMakeLists.txt:8 (enable_language):
  The CMAKE_ASM_COMPILER:

    gcc

  is not a full path and was not found in the PATH.

  Tell CMake where to find the compiler by setting either the environment
  variable "ASM" or the CMake cache entry CMAKE_ASM_COMPILER to the full path
  to the compiler, or to the compiler name if it is in the PATH.

even though gcc is in the PATH. I'm opening this issue following !2418 (merged).