Masquerade as Linux to enable certain build options.
Always use pthread even if feature detection is disabled.

--- GNUmakefile.orig	2022-08-07 19:52:03 UTC
+++ GNUmakefile
@@ -70,7 +70,7 @@ endif
   SYSTEMX := $(shell uname -s 2>/dev/null)
 endif
 
-IS_LINUX := $(shell echo "$(SYSTEMX)" | $(GREP) -i -c "Linux")
+IS_LINUX := $(shell echo "$(SYSTEMX)" | $(GREP) -i -c -E "Linux|FreeBSD")
 IS_HURD := $(shell echo "$(SYSTEMX)" | $(GREP) -i -c -E "GNU|Hurd")
 IS_MINGW := $(shell echo "$(SYSTEMX)" | $(GREP) -i -c "MinGW")
 IS_CYGWIN := $(shell echo "$(SYSTEMX)" | $(GREP) -i -c "Cygwin")
@@ -848,7 +848,7 @@ endif
 
 # Use -pthread whenever it is available. See http://www.hpl.hp.com/techreports/2004/HPL-2004-209.pdf
 #   http://stackoverflow.com/questions/2127797/gcc-significance-of-pthread-flag-when-compiling
-ifeq ($(DETECT_FEATURES),1)
+ifeq (1,1)
  ifeq ($(XLC_COMPILER),1)
   ifeq ($(findstring -qthreaded,$(CXXFLAGS)),)
    TPROG = TestPrograms/test_pthreads.cpp