make test-gpp, error: 'xxx' is not a member of 'std'

Description

When using Intel® HLS 18.1 or later, you may encounter the following error during compiling HLS examples by "make test-gpp":

"error:'xxx' is not a member of 'std' "

Such error is due to C 11 / C 14 feature is used in HLS code but C 11 / C 14 option is not enabled in makefile

Resolution

Add c 14 support in make file, for example:

test-gpp: CXX := g -std=c 11

test-gpp: CXX := g -std=c 14