libjpeg-turbo: set compiler flags with CMake args (#11938)

Later versions of libjpeg-turbo build with CMake; to build with
user-specified cflags, the user must supply these to CMake as
-DCMAKE_C_FLAGS (Spack's typical approach of injecting these flags
into the compiler wrapper invocation is insufficient in this case).

Currently libjpeg-turbo cannot be implemented as a CMakePackage
(and thereby take advantage of the flag_handler implementation it
provides) because not all versions of libjpeg-turbo use CMake, so
this adds a custom implementation of flag_handler and
flags_to_build_system_args to libjpeg-turbo.

var/spack/repos/builtin/packages/libjpeg-turbo/package.py

@@ -38,6 +38,22 @@ class LibjpegTurbo(Package):
     def libs(self):
         return find_libraries("libjpeg*", root=self.prefix, recursive=True)
 
+    def flag_handler(self, name, flags):
+        if self.spec.satisfies('@1.5.90:'):
+            return (None, None, flags)
+        else:
+            # compiler flags for earlier version are injected into the
+            # spack compiler wrapper
+            return (flags, None, None)
+
+    def flags_to_build_system_args(self, flags):
+        # This only handles cflags, other flags are discarded
+        cmake_flag_args = []
+        if 'cflags' in flags and flags['cflags']:
+            cmake_flag_args.append('-DCMAKE_C_FLAGS={0}'.format(
+                                   ' '.join(flags['cflags'])))
+        self.cmake_flag_args = cmake_flag_args
+
     @when('@1.3.1:1.5.3')
     def install(self, spec, prefix):
         autoreconf('-ifv')
@@ -48,6 +64,8 @@ def install(self, spec, prefix):
     @when('@1.5.90:')
     def install(self, spec, prefix):
         cmake_args = ['-GUnix Makefiles']
+        if hasattr(self, 'cmake_flag_args'):
+            cmake_args.extend(self.cmake_flag_args)
         cmake_args.extend(std_cmake_args)
         with working_dir('spack-build', create=True):
             cmake('..', *cmake_args)