Detect OpenMP support

This detects support for OpenMP and only includes the compiler flags for
it if OpenMP is supported.

This solves the common case on macOS, because R on macOS ships with
`-fopenmp` in SHLIB_OPENMP_CFLAGS, as the CRAN compiler toolchain _does_
support OpenMP, however the XCode toolchain distributed by Apple _does
 not_ support OpenMP, which causes data.table to fail to compile from
 source.

Fixes Rdatatable#2161

.Rbuildignore

@@ -15,6 +15,7 @@
 ^NEWS\.0\.md$
 ^README\.md$
 ^_pkgdown\.yml$
+^src/Makevars$
 
 ^\.RData$
 ^\.Rhistory$

.gitignore

@@ -9,6 +9,7 @@ Rplots.pdf
 *-Ex.R
 data.table_*.tar.gz
 data.table.Rcheck
+src/Makevars
 
 # Emacs IDE files
 .emacs.desktop

NEWS.md

@@ -6,6 +6,10 @@
 
 ## NEW FEATURES
 
+* Compiler support for OpenMP is now detected during installation, which allows
+  data.table to compile even if the users' toolchain differs from CRANs, as is
+  common on macOS. (#2161, @jimhester)
+
 ## BUG FIXES
 
 ## NOTES

cleanup

@@ -0,0 +1,2 @@
+#!/bin/sh
+rm -f src/Makevars

configure

@@ -50,5 +50,22 @@ fi
 
 version=`pkg-config --modversion zlib`
 echo "zlib ${version} is available ok"
-exit 0
 
+# Find R compilers
+CC=`${R_HOME}/bin/R CMD config CC`
+CFLAGS=`${R_HOME}/bin/R CMD config CFLAGS`
+CPPFLAGS=`${R_HOME}/bin/R CMD config CPPFLAGS`
+
+# Test if we have a OPENMP compatible compiler
+echo "#include <omp.h>\nint main () { return omp_get_num_threads (); }" | ${CC} ${CPPFLAGS} ${PKG_CFLAGS} ${CFLAGS} ${SHLIB_OPENMP_CFLAGS} -E -xc - >/dev/null 2>&1 || R_NO_OPENMP=1;
+
+# Write to Makevars
+if [ $R_NO_OPENMP ]; then
+  sed -e "s|@openmp_cflags@||" src/Makevars.in > src/Makevars
+else
+  echo "OpenMP supported"
+  sed -e "s|@openmp_cflags@|\$(SHLIB_OPENMP_CFLAGS)|" src/Makevars.in > src/Makevars
+fi
+
+
+exit 0

src/Makevars → src/Makevars.in

@@ -1,9 +1,6 @@
-
-PKG_CFLAGS = $(SHLIB_OPENMP_CFLAGS)
-PKG_LIBS = $(SHLIB_OPENMP_CFLAGS) -lz
+PKG_CFLAGS = @openmp_cflags@
+PKG_LIBS = @openmp_cflags@ -lz
 
 all: $(SHLIB)
 	mv $(SHLIB) datatable$(SHLIB_EXT)
 	if [ "$(OS)" != "Windows_NT" ] && [ `uname -s` = 'Darwin' ]; then install_name_tool -id datatable$(SHLIB_EXT) datatable$(SHLIB_EXT); fi
-
-

src/Makevars.win

@@ -0,0 +1,5 @@
+PKG_CFLAGS = $(SHLIB_OPENMP_CFLAGS)
+PKG_LIBS = $(SHLIB_OPENMP_CFLAGS) -lz
+
+all: $(SHLIB)
+	mv $(SHLIB) datatable$(SHLIB_EXT)