GCC<=4.9: Exceptions

[ax3l]

GCC 4.8 and 4.9 support exceptions, but are not detected as such. This adds exception support for those compilers.

Detection taken from nlohmann/json:

• https://github.com/nlohmann/json/issues/498
• https://github.com/nlohmann/json/blob/v3.9.1/benchmarks/thirdparty/benchmark/src/internal_macros.h#L70-L73

• https://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html

  __EXCEPTIONS
  
  This macro is defined, with value 1, when compiling a C++ source file with exceptions enabled. If -fno-exceptions is used when compiling the file, then this macro is not defined.

[ax3l]

@mpark friendly ping :)

[ax3l]

@mpark friendly reminder :)

[mpark]

Sorry for the delay. Merged.