libidn 1.36 with non-GNU make

[Bruno Haible]

On glibc systems I build with GNU make, of course.
On BSD systems and Solaris, I build with GNU make, because the native 'make'
has problem with VPATH builds.
On AIX, on the other hand, the native 'make' is good enough for most GNU
packages. But it causes a build failure in libidn-1.36. This is what I see
on AIX 7.1:

$ ../configure...
...
configure: summary of build options:

  version:           1.36 shared 18:1:6
  Host type:         powerpc-ibm-aix7.1.3.0
  Install prefix:    /home/haible/prefix32
  Compiler:          xlc -qthreaded -qtls
  Compiler flags:    -g
  Warning flags:     
  Library types:     Shared=yes, Static=no
  Libiconv:          yes
  Valgrind:          
  Version script:    no
  Documentation:     yes
  TLD code:          yes
  Build Java port:   no
  Build C# port:     no
  Address Sanitizer: no
  UB Sanitizer:      no

$ make
"Makefile", line 2070: make: 1254-055 Dependency line needs colon or double 
colon operator.
"Makefile", line 2071: make: 1254-055 Dependency line needs colon or double 
colon operator.
make: 1254-058 Fatal errors encountered -- cannot continue.

This syntax is not understood by 'make':
COVERAGE_CCOPTS ?= "-g --coverage"
COVERAGE_OUT ?= doc/coverage
I removed the question marks; then the build proceeded.

Bruno