checking for GNU extensions in Makefiles ... WARNING

I added the following to my Makevars file:

# Set the library flags based on the operating system
ifeq ($(findstring linux,$(OSTYPE)), linux)
    PKG_LIBS = -L$(LIBDIR) -lharmonium -lasound
else
    PKG_LIBS = -L$(LIBDIR) -lharmonium
endif

And devtools::check is complaining:

❯ checking for GNU extensions in Makefiles ... WARNING
  Found the following file(s) containing GNU extensions:
    src/Makevars
  Portable Makefiles do not use GNU extensions such as +=, :=, $(shell),
  $(wildcard), ifeq ... endif, .NOTPARALLEL See section ‘Writing portable
  packages’ in the ‘Writing R Extensions’ manual.

How to avoid this warning while keeping Makevars dependent on $OSTYPE?

EDIT: this is the solution I got following Dirk's advice.

PKG_LIBS = `os=\`uname -s\`; if [ "$$os" = "Linux" ]; then echo "-L${LIBDIR} -lharmonium -lasound"; else echo ="-L${LIBDIR} -lharmonium"; fi`

You could call grep directly and use the shell variable holding the value of the last command to access is. However, a more common approach is to query uname -s and compare. I use both approaches here.

Code

#!/bin/sh

echo $OSTYPE | grep -q linux
if [ $? -eq 0 ]; then
    echo "We are on Linux (1 of 2)"
    echo "PKG_LIBS = -L${LIBDIR} -lharmonium -lasound"
fi

os=`uname -s`
if [ "$os" = "Linux" ]; then
    echo "We are on Linux (2 of 2)"
    echo "PKG_LIBS = -L${LIBDIR} -lharmonium -lasound"
fi

Output

While I see OSTYPE under bash, it is empty for me in sh making only the second approach viable.

$ ./answer.sh 
We are on Linux (2 of 2)
PKG_LIBS = -L -lharmonium -lasound
$ 

In your real use you want to of course assign to PKG_LIBS rather than echo it.