build: Add support for OpenMP dependency

Enables general support for OpenMP.
Currently, only the DDS plug-in utilizes it.

configure.ac

@@ -1373,6 +1373,17 @@ if test "x$enable_mp" != "xno"; then
 	    [Define to 1 to enable support for multiple processors.])
 fi
 
+enable_openmp=no
+
+m4_ifdef([AC_OPENMP], [AC_OPENMP], [AX_OPENMP([enable_openmp=yes],[enable_openmp=no])])
+if test "x$ac_cv_prog_c_openmp" != "xunsupported" && test "x$ac_cv_prog_c_openmp" != "x"; then
+	enable_openmp=yes
+fi
+
+if test "x$enable_openmp" = "xyes"; then
+    AC_DEFINE(USE_OPENMP, 1, [Define to 1 to enable support for OpenMP])
+ 	CFLAGS="$CFLAGS $(OPENMP_CFLAGS)"
+fi
 
 ################################
 # Some plug-ins are X11 specific

meson.build

@@ -1256,6 +1256,15 @@ conf.set('HAVE_GETTEXT', true)
 # Enable support for multiprocessing
 conf.set10('ENABLE_MP', get_option('enable-multiproc'))
 
+# Enable support for OpenMP
+openmp = dependency('openmp', required : false)
+if openmp.found()
+  have_openmp = true
+else
+  have_openmp = false
+endif
+conf.set('HAVE_OPENMP', have_openmp)
+
 # Check for available functions
 foreach fn : [
     { 'm': 'HAVE_ALLOCA',                   'v': 'alloca', },
@@ -1831,6 +1840,7 @@ final_message = [
 '''  32-bit DLL folder (Win32): @0@'''.format(get_option('win32-32bits-dll-folder')),
 '''  Detailed backtraces:       @0@'''.format(detailed_backtraces),
 '''  Binary symlinks:           @0@'''.format(enable_default_bin),
+'''  OpenMP:                    @0@'''.format(have_openmp),
 '',
 '''Optional Plug-Ins:''',
 '''  Ascii Art:           @0@'''.format(libaa.found()),

plug-ins/file-dds/Makefile.am

@@ -20,7 +20,7 @@ file_dds_RC = file-dds.rc.o
 endif
 
 AM_LDFLAGS = $(mwindows)
-AM_CFLAGS = -fno-strict-aliasing
+AM_CFLAGS = -fno-strict-aliasing -fopenmp
 
 libexecdir = $(gimpplugindir)/plug-ins/file-dds
 

plug-ins/file-dds/meson.build

@@ -29,6 +29,7 @@ executable(plugin_name,
   dependencies: [
     libgimpui_dep,
     math,
+    openmp,
   ],
   install: true,
   install_dir: gimpplugindir / 'plug-ins' / plugin_name,