GNU make - How to concatenate to variable depending on shell command result (GCC version)?

Question

I want to add another option to the CFLAGS make variable, depending on the result of a shell command that i want to execute outside of a recipe in my "configuration" section of the makefile. This is what i have come up with:

GCC_VERSION := $(shell gcc -dumpversion); \
if [[ ${GCC_VERSION} > 5.0 ]] ; then \
    CFLAGS += -D _POSIX_C_SOURCE=199309L; \
fi

At first i execute the command with the shell make function as you see above. If i execute the above it doesn't add this define flag. I intentionally do this on linux with GCC Version 5.4.0. I believe this is wrong because then i have to create a new shell to execute the conditional statement. In that new shell though the GCC_VERSION variable will not exist. I could be wrong though.

If i do like this then (all in one shell):

$(shell GCC_VERSION=$(gcc -dumpversion); \
if [[ ${GCC_VERSION} > 5.0 ]] ; then \
    CFLAGS += -D _POSIX_C_SOURCE=199309L; \
fi)

i get error:

*** recipe commences before first target.  Stop.

Yeah, very confusing.

If someone could help i would appreciate it. Thanks.

Answer 1

There are many solutions, including this one. In your Makefile use

VERSION5 := $(shell \
 GCC_VERSION=$$(gcc -dumpversion); \
 [[ $$GCC_VERSION > 5.0 ]]; \
 echo $$? )

ifeq (${VERSION5}, 0)
  CFLAGS += -D _POSIX_C_SOURCE=199309L
endif

Note in particular, that you need to use $$ for every $ in your shell script. This shell echos 0 if the string comparision with 5.0 is true, else 1, and this is saved in make variable VERSION5.

Then the ifeq test in the Makefile compares the variable with 0 and if it matches edits the CFLAGS variable.