Wanted: Workaround for deduction guide detection on gcc

[HDembinski]

gcc-7 and gcc-8 both fail the deduction guide detection check.
gcc-7 (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0
gcc-8 (Ubuntu 8.2.0-1ubuntu2~18.04) 8.2.0

g++-7 -std=c++17 ../config/checks/std/cpp_deduction_guides_17.cpp 
../config/checks/std/cpp_deduction_guides_17.cpp:23:2: error: #error "Macro __cpp_deduction_guides had too low a value"
 #error "Macro __cpp_deduction_guides had too low a value"

g++-8 -std=c++17 ../config/checks/std/cpp_deduction_guides_17.cpp 
../config/checks/std/cpp_deduction_guides_17.cpp:23:2: error: #error "Macro __cpp_deduction_guides had too low a value"
 #error "Macro __cpp_deduction_guides had too low a value"

These compilers support deduction guides, but the value for __cpp_deduction_guides is 201606 or 201611, respectively.
https://gcc.gnu.org/projects/cxx-status.html#cxx17

Perhaps the threshold value can be lowered to 201611?

[jzmaddock]

Done, I'm a bit concerned that there are corner cases that the lower threshold won't handle, but we'll see.

[HDembinski]

I hope it is going to be ok, thank you for merging.