https://sourceware.org/git/?p=glibc.git;a=commit;h=fc3e1337be1c6935ab58bd13520f97a535cf70cc

commit dc23a45db566095e83ff0b7a57afc87fb5ca89a1
Author: Florian Weimer <fweimer@redhat.com>
Date:   Wed Sep 21 10:45:32 2016 +0200

    Avoid running $(CXX) during build to obtain header file paths
    
    This reduces the build time somewhat and is particularly noticeable
    during rebuilds with few code changes.

diff --git a/Makerules b/Makerules
index 7e4077ee50..c338850de5 100644
--- a/Makerules
+++ b/Makerules
@@ -121,14 +121,10 @@ ifneq (,$(CXX))
 # will be used instead of /usr/include/stdlib.h and /usr/include/math.h.
 before-compile := $(common-objpfx)cstdlib $(common-objpfx)cmath \
 	          $(before-compile)
-cstdlib=$(shell echo "\#include <cstdlib>" | $(CXX) -M -MP -x c++ - \
-		| sed -n "/cstdlib:/{s/:$$//;p}")
-$(common-objpfx)cstdlib: $(cstdlib)
+$(common-objpfx)cstdlib: $(c++-cstdlib-header)
 	$(INSTALL_DATA) $< $@T
 	$(move-if-change) $@T $@
-cmath=$(shell echo "\#include <cmath>" | $(CXX) -M -MP -x c++ - \
-		| sed -n "/cmath:/{s/:$$//;p}")
-$(common-objpfx)cmath: $(cmath)
+$(common-objpfx)cmath: $(c++-cmath-header)
 	$(INSTALL_DATA) $< $@T
 	$(move-if-change) $@T $@
 endif
diff --git a/config.make.in b/config.make.in
index 95c6f36876..04a8b3ed7f 100644
--- a/config.make.in
+++ b/config.make.in
@@ -45,6 +45,8 @@ defines = @DEFINES@
 sysheaders = @sysheaders@
 sysincludes = @SYSINCLUDES@
 c++-sysincludes = @CXX_SYSINCLUDES@
+c++-cstdlib-header = @CXX_CSTDLIB_HEADER@
+c++-cmath-header = @CXX_CMATH_HEADER@
 all-warnings = @all_warnings@
 enable-werror = @enable_werror@
 
diff --git a/configure b/configure
index 17625e1041..6ff252744b 100755
--- a/configure
+++ b/configure
@@ -635,6 +635,8 @@ BISON
 INSTALL_INFO
 PERL
 BASH_SHELL
+CXX_CMATH_HEADER
+CXX_CSTDLIB_HEADER
 CXX_SYSINCLUDES
 SYSINCLUDES
 AUTOCONF
@@ -5054,6 +5056,18 @@ fi
 
 
 
+# Obtain some C++ header file paths.  This is used to make a local
+# copy of those headers in Makerules.
+if test -n "$CXX"; then
+  find_cxx_header () {
+    echo "#include <$1>" | $CXX -M -MP -x c++ - | sed -n "/$1:/{s/:\$//;p}"
+  }
+  CXX_CSTDLIB_HEADER="$(find_cxx_header cstdlib)"
+  CXX_CMATH_HEADER="$(find_cxx_header cmath)"
+fi
+
+
+
 # Test if LD_LIBRARY_PATH contains the notation for the current directory
 # since this would lead to problems installing/building glibc.
 # LD_LIBRARY_PATH contains the current directory if one of the following
diff --git a/configure.ac b/configure.ac
index 33bcd62180..9938ab0dc2 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1039,6 +1039,18 @@ fi
 AC_SUBST(SYSINCLUDES)
 AC_SUBST(CXX_SYSINCLUDES)
 
+# Obtain some C++ header file paths.  This is used to make a local
+# copy of those headers in Makerules.
+if test -n "$CXX"; then
+  find_cxx_header () {
+    echo "#include <$1>" | $CXX -M -MP -x c++ - | sed -n "/$1:/{s/:\$//;p}"
+  }
+  CXX_CSTDLIB_HEADER="$(find_cxx_header cstdlib)"
+  CXX_CMATH_HEADER="$(find_cxx_header cmath)"
+fi
+AC_SUBST(CXX_CSTDLIB_HEADER)
+AC_SUBST(CXX_CMATH_HEADER)
+
 # Test if LD_LIBRARY_PATH contains the notation for the current directory
 # since this would lead to problems installing/building glibc.
 # LD_LIBRARY_PATH contains the current directory if one of the following