Add C++ support to configure.

This is an optional dependency. It'll be used for the upcoming LLVM
based just in time compilation support, which needs to wrap a few LLVM
C++ APIs so they're accessible from C..

For now test for C++ compilers unconditionally, without failing if not
present, to ensure wide buildfarm coverage. If we're bothered by the
additional test times (which are quite short) or verbosity, we can
later make the tests conditional on --with-llvm.

Author: Andres Freund
Discussion: https://postgr.es/m/20170901064131.tazjxwus3k2w3ybh@alap3.anarazel.de

src/Makefile.global.in

@@ -249,9 +249,11 @@ endif # not PGXS
 CC = @CC@
 GCC = @GCC@
 SUN_STUDIO_CC = @SUN_STUDIO_CC@
+CXX = @CXX@
 CFLAGS = @CFLAGS@
 CFLAGS_VECTOR = @CFLAGS_VECTOR@
 CFLAGS_SSE42 = @CFLAGS_SSE42@
+CXXFLAGS = @CXXFLAGS@
 
 # Kind-of compilers
 
@@ -813,6 +815,10 @@ ifndef COMPILE.c
 COMPILE.c = $(CC) $(CFLAGS) $(CPPFLAGS) -c
 endif
 
+ifndef COMPILE.cc
+COMPILE.cc = $(CXX) $(CXXFLAGS) $(CPPFLAGS) -c
+endif
+
 DEPDIR = .deps
 
 ifeq ($(GCC), yes)
@@ -822,6 +828,10 @@ ifeq ($(GCC), yes)
 	@if test ! -d $(DEPDIR); then mkdir -p $(DEPDIR); fi
 	$(COMPILE.c) -o $@ $< -MMD -MP -MF $(DEPDIR)/$(*F).Po
 
+%.o : %.cpp
+	@if test ! -d $(DEPDIR); then mkdir -p $(DEPDIR); fi
+	$(COMPILE.cc) -o $@ $< -MMD -MP -MF $(DEPDIR)/$(*F).Po
+
 endif # GCC
 
 # Include all the dependency files generated for the current

src/Makefile.shlib

@@ -100,6 +100,7 @@ endif
 # Try to keep the sections in some kind of order, folks...
 
 override CFLAGS += $(CFLAGS_SL)
+override CXXFLAGS += $(CFLAGS_SL)
 ifdef SO_MAJOR_VERSION
 # libraries ought to use this to refer to versioned gettext domain names
 override CPPFLAGS += -DSO_MAJOR_VERSION=$(SO_MAJOR_VERSION)