os = $(uname -a | awk '{print $2}')
ifeq(os, "redhat7")
    CC=/opt/rh/devtoolset-7/root/usr/bin/gcc
    CXX=/opt/rh/devtoolset-7/root/usr/bin/g++
    ...
endif

include gmtt-master/gmtt-master/gmtt.mk

# Table with two (indicator "2" as first element) columns. First column is a glob which matches the OS, second is option string
# Warning! No spaces in either column are allowed - use $(call spc-mask,...options...) if there are.
define CC_OPTIONS_TBL
2
redhat[!7]* /opt/rh/devtoolset-6/root/usr/bin/gcc
redhat7     /opt/rh/devtoolset-7/root/usr/bin/gcc
suse*       /opt/bin/rh/devtoolset-7/root/usr/bin/gcc
endef

CC_OPT := $(call select,2,$(CC_OPTIONS_TBL),$$(call glob-match,$(MY_OS),$$1))

$(info CC_OPT = $(CC_OPT))

define CXX_OPTIONS_TBL
2
redhat[!7]* $(call spc-mask, /opt/rh/devtoolset-6/root/usr/bin/gcc -DREDHAT -O2)
redhat7     $(call spc-mask, /opt/rh/devtoolset-7/root/usr/bin/gcc -DREDHAT_7 -O3)
suse*       $(call spc-mask, /opt/bin/rh/devtoolset-7/root/usr/bin/gcc -DSUSE -O0)
suse8       $(call spc-mask, -DFOO -DBAR)
endef

CXX_OPT := $(call spc-unmask,$(call select,2,$(CXX_OPTIONS_TBL),$$(call glob-match,$(MY_OS),$$1)))

$(info CXX_OPT = $(CXX_OPT))

mark@zws103 ~
$ make MY_OS=suse
CC_OPT =  /opt/bin/rh/devtoolset-7/root/usr/bin/gcc
CXX_OPT =   /opt/bin/rh/devtoolset-7/root/usr/bin/gcc -DSUSE -O0
make: *** Keine Ziele.  Schluss.

mark@zws103 ~
$ make MY_OS=redhat6.1
CC_OPT =  /opt/rh/devtoolset-6/root/usr/bin/gcc
CXX_OPT =   /opt/rh/devtoolset-6/root/usr/bin/gcc -DREDHAT -O2
make: *** Keine Ziele.  Schluss.

mark@zws103 ~
$ make MY_OS=redhat7
CC_OPT =  /opt/rh/devtoolset-7/root/usr/bin/gcc
CXX_OPT =   /opt/rh/devtoolset-7/root/usr/bin/gcc -DREDHAT_7 -O3
make: *** Keine Ziele.  Schluss.

mark@zws103 ~
$ make MY_OS=redhat7.1
CC_OPT =
CXX_OPT =
make: *** Keine Ziele.  Schluss.

$ make MY_OS=suse8
CC_OPT =  /opt/bin/rh/devtoolset-7/root/usr/bin/gcc
CXX_OPT =   /opt/bin/rh/devtoolset-7/root/usr/bin/gcc -DSUSE -O0  -DFOO -DBAR
make: *** Keine Ziele.  Schluss.