Building R-devel from source

Hello,

I'm (for the first time) trying to build R-devel from source (to be able 
to check my CRAN packages on a Mac), but it breaks already after

gobr0002 at MBP-2:~/R/src/R-devel$ ./configure

with

checking size of size_t... configure: error: in `/Users/gb/R/src/R-devel':
configure: error: cannot compute sizeof (size_t)
See `config.log' for more details

Some 'details' regarding size_t:

configure:23748: checking for size_t
configure:23748: gcc -c -g -O2 -I/usr/local/include conftest.c >&5
configure:23748: $? = 0
configure:23748: gcc -c -g -O2 -I/usr/local/include conftest.c >&5
conftest.c: In function 'main':
conftest.c:116:21: error: expected expression before ')' token
  if (sizeof ((size_t)))
                      ^

My MacBook Pro is old (2010), so I am stuck with MacOS 10.13.6, XCode 
10.1. I'm using clang 7.0.0 and fortran 6.1.0.

Do I need to buy a new Mac?

Thanks, G?ran