Bug fix for glibc broke freebsd build in REL_11_STABLE

Collegues,

Few days ago commit 

 1f349aa7d9a6633e87db071390c73a39ac279ba4

Fix 8a934d67 for libc++ and make more include order resistant. 

was introduced into branch REL_11_STABLE.
It seems that it deals with GLIBC-based systems (i.e Linux and Hurd)
only, and shouldn't affect systems with non-GNU libc (such as BSD
family).

It changes code  which has a comment:

/*
 * Glibc doesn't use the builtin for clang due to a *gcc* bug in a
version
 * newer than the gcc compatibility clang claims to have. This would
cause a
 * *lot* of superfluous function calls, therefore revert when using
clang. In
 * C++ there's issues with libc++ (not libstdc++), so disable as well.
 */


However, this commit broke float8 test on 32-bit FreeBSD 11 with clang
3.8.0 compiler. Regressions.diff follows:

================== pgsql.build/src/test/regress/regression.diffs
===================
*** /usr/home/buildfarm/build-farm/REL_11_STABLE/pgsql.build/src/test/regress/expected/float8.out
Tue Sep  4 11:57:47 2018
--- /usr/home/buildfarm/build-farm/REL_11_STABLE/pgsql.build/src/test/regress/results/float8.out
Tue Sep  4 12:03:28 2018 *************** *** 418,424 **** SET f1 =
FLOAT8_TBL.f1 * '-1' WHERE FLOAT8_TBL.f1 > '0.0';
  SELECT '' AS bad, f.f1 * '1e200' from FLOAT8_TBL f;
! ERROR:  value out of range: overflow
  SELECT '' AS bad, f.f1 ^ '1e200' from FLOAT8_TBL f;
  ERROR:  value out of range: overflow
  SELECT 0 ^ 0 + 0 ^ 1 + 0 ^ 0.0 + 0 ^ 0.5;
--- 418,432 ----
     SET f1 = FLOAT8_TBL.f1 * '-1'
     WHERE FLOAT8_TBL.f1 > '0.0';
  SELECT '' AS bad, f.f1 * '1e200' from FLOAT8_TBL f;
!  bad |     ?column?     
! -----+------------------
!      |                0
!      |      -3.484e+201
!      |     -1.0043e+203
!      |        -Infinity
!      | -1.2345678901234
! (5 rows)
! 
  SELECT '' AS bad, f.f1 ^ '1e200' from FLOAT8_TBL f;
  ERROR:  value out of range: overflow
  SELECT 0 ^ 0 + 0 ^ 1 + 0 ^ 0.0 + 0 ^ 0.5; 
================================================================
Problem doesn't arise on 64-bit FreeBSD and MacOS. (we have no 32-bit
MacOs at hand).

Following patch fixes this problem:

diff --git a/src/include/port.h b/src/include/port.h
index 0ce72e50e5..7daaebf568 100644
--- a/src/include/port.h
+++ b/src/include/port.h
@@ -350,7 +350,7 @@ extern int  isinf(double x);
  * *lot* of superfluous function calls, therefore revert when using
clang. In
  * C++ there's issues with libc++ (not libstdc++), so disable as well.
  */
-#if defined(__clang__) && !defined(__cplusplus)
+#if defined(__clang__) && !defined(__cplusplus) && defined(__linux__)
 /* needs to be separate to not confuse other compilers */
 #if __has_builtin(__builtin_isinf)
 /* need to include before, to avoid getting overwritten */

Idea of the patch is that because patch is only GLIBC-related, we
should apply this logic only if we have linux (more precisely linux or
hurd, but adding hurd would make condition overcomplicated and no one
seems to use it) system.
--