bpo-33473: Be slightly better about CFLAGS, LDFLAGS, and related

- "-pthread" is a part of the C compiler and linker flags. It is not
part of the compiler. In particular 'make CC=clang' ought to work
- checking for pthreads support is a link time issue, not run time

Not done in this patch:
- convert from "gcc" to "cc" to more correctly infer the current
compiler
- regenerate generated code such as "configure" or "aclocal.m4"

Author: grimreaper

Misc/NEWS.d/next/Build/2018-05-12-15-58-15.bpo-33473.BEm_Wt.rst

@@ -0,0 +1 @@
+configure.ac was fixed to correctly report CC, CFLAGS, and LDFLAGS

configure.ac

@@ -1993,8 +1993,9 @@ then
 AC_MSG_CHECKING(whether $CC accepts -pthread)
 AC_CACHE_VAL(ac_cv_pthread,
 [ac_save_cc="$CC"
-CC="$CC -pthread"
-AC_RUN_IFELSE([AC_LANG_SOURCE([[
+CC="$CC"
+LDFLAGS="-pthread"
+AC_LINK_IFELSE([AC_LANG_SOURCE([[
 #include <stdio.h>
 #include <pthread.h>
 
@@ -2030,7 +2031,8 @@ then
   ac_cv_cxx_thread=yes
 elif test "$ac_cv_pthread" = "yes"
 then
-  CXX="$CXX -pthread"
+  CXX="$CXX"
+  CXXFLAGS="-pthread"
   ac_cv_cxx_thread=yes
 fi
 
@@ -2261,7 +2263,7 @@ then CC="$CC -Kpthread"
 elif test "$ac_cv_kthread" = "yes"
 then CC="$CC -Kthread"
 elif test "$ac_cv_pthread" = "yes"
-then CC="$CC -pthread"
+then CC="$CC"; BASECFLAGS="-pthread"; BASECXXFLAGS="-pthread"
 fi
 
 AC_MSG_CHECKING(for pthread_t)
@@ -2973,9 +2975,11 @@ then
     posix_threads=yes
 elif test "$ac_cv_pthread" = "yes"
 then
-    CC="$CC -pthread"
+    CC="$CC"
+    BASECFLAGS="-pthread"
     if test "$ac_cv_cxx_thread" = "yes"; then
-        CXX="$CXX -pthread"
+        CXX="$CXX"
+	BASECXXFLAGS="-pthread"
     fi
     posix_threads=yes
 else