correct logic bug in else branch of the conditional

lib/python/Makefile.lammps.python2

 # Settings that the LAMMPS build will import when this package library is used
 # See the README file for more explanation
 
-python_SYSINC = $(shell which python2-config > /dev/null 2>&1 && python2-config --includes || which python-config > /dev/null 2>&1 && python-config --includes || :)
-python_SYSLIB = $(shell which python2-config > /dev/null 2>&1 && python2-config --ldflags || which python-config > /dev/null 2>&1 && python-config --ldflags || :)
+python_SYSINC = $(shell which python2-config > /dev/null 2>&1 && python2-config --includes || (which python-config > /dev/null 2>&1 && python-config --includes || :))
+python_SYSLIB = $(shell which python2-config > /dev/null 2>&1 && python2-config --ldflags || (which python-config > /dev/null 2>&1 && python-config --ldflags || :))
 python_SYSPATH =
 PYTHON=$(shell which python2 > /dev/null 2>&1 && echo python2 || echo python)

lib/python/Makefile.lammps.python3

 # Settings that the LAMMPS build will import when this package library is used
 # See the README file for more explanation
 
-python_SYSINC = $(shell which python3-config > /dev/null 2>&1 && python3-config --includes || which python-config > /dev/null 2>&1 && python-config --includes || :)
-python_SYSLIB = $(shell which python3-config > /dev/null 2>&1 && python3-config --ldflags || which python-config > /dev/null 2>&1 && python-config --ldflags || : )
+python_SYSINC = $(shell which python3-config > /dev/null 2>&1 && python3-config --includes || (which python-config > /dev/null 2>&1 && python-config --includes || :))
+python_SYSLIB = $(shell which python3-config > /dev/null 2>&1 && python3-config --ldflags || (which python-config > /dev/null 2>&1 && python-config --ldflags || :))
 python_SYSPATH =
 PYTHON=$(shell which python3 > /dev/null 2>&1 && echo python3 || echo python)