Make building safestringlib on some distros easier (#3616)

On some distros (e.g. Redhat 7) there is cmake version 2 and cmake version 3,
safestringlib requires cmake version 3. On those distros the binary is called
cmake3, so try to use that one before falling back to regular cmake binary.

(cherry picked from commit 99c5b0add7)

src/backend/distributed/Makefile

@@ -7,6 +7,13 @@ safestringlib_builddir = $(citus_top_builddir)/vendor/safestringlib/build
 safestringlib_a = $(safestringlib_builddir)/libsafestring_static.a
 safestringlib_sources = $(wildcard $(safestringlib_srcdir)/safeclib/*)
 
+# Use cmake3 if it exists, otherwise just use the string "cmake" and let bash
+# figure out its exact path later.
+# Using "which cmake" instead will make the error very hard to understand if
+# you have neither "cmake3" or "cmake" in path. In that case the "CMAKE3"
+# variable would be an empty string.
+CMAKE3 = $(shell which cmake3 || echo cmake)
+
 MODULE_big = citus
 EXTENSION = citus
 
@@ -56,7 +63,7 @@ $(safestringlib_a): $(safestringlib_sources)
 	@# Makefile are not used
 	+cd $(safestringlib_builddir) && \
 		export LDFLAGS='' && export CPPFLAGS='' && \
-		cmake $(safestringlib_srcdir) && make
+		$(CMAKE3) $(safestringlib_srcdir) && make
 
 citus.so: $(safestringlib_a)