Don't assume cflags is non-nil

Fixes Homebrew/homebrew#29284.

Formula/fftw.rb

@@ -73,7 +73,7 @@ class Fftw < Formula
       }
     TEST_SCRIPT
 
-    system ENV.cc, '-o', 'fftw', 'fftw.c', '-lfftw3', *ENV.cflags.split
+    system ENV.cc, '-o', 'fftw', 'fftw.c', '-lfftw3', *ENV.cflags.to_s.split
     system './fftw'
   end
 end

Formula/glib.rb

@@ -114,7 +114,7 @@ class Glib < Formula
       }
       EOS
     flags = ["-I#{include}/glib-2.0", "-I#{lib}/glib-2.0/include", "-lglib-2.0"]
-    system ENV.cc, "-o", "test", "test.c", *(flags + ENV.cflags.split)
+    system ENV.cc, "-o", "test", "test.c", *(flags + ENV.cflags.to_s.split)
     system "./test"
   end
 end

Formula/hidapi.rb

@@ -25,7 +25,7 @@ class Hidapi < Formula
       }
     EOS
 
-    flags = `pkg-config --cflags --libs hidapi`.split + ENV.cflags.split
+    flags = `pkg-config --cflags --libs hidapi`.split + ENV.cflags.to_s.split
     system ENV.cc, "-o", "test", "test.c", *flags
     system './test'
   end

Formula/libcapn.rb

@@ -30,7 +30,7 @@ class Libcapn < Formula
         return 0;
     }
     TEST_SCRIPT
-    flags = `#{HOMEBREW_PREFIX}/bin/pkg-config --cflags --libs libcapn`.split + ENV.cflags.split
+    flags = `#{HOMEBREW_PREFIX}/bin/pkg-config --cflags --libs libcapn`.split + ENV.cflags.to_s.split
     system ENV.cc, "-o", "test_install", "test_install.c", *flags
     system "./test_install"
   end

Formula/libffi.rb

@@ -74,7 +74,7 @@ class Libffi < Formula
     TEST_SCRIPT
 
     flags = ["-L#{lib}", "-lffi", "-I#{lib}/libffi-#{version}/include"]
-    system ENV.cc, "-o", "closure", "closure.c", *(flags + ENV.cflags.split)
+    system ENV.cc, "-o", "closure", "closure.c", *(flags + ENV.cflags.to_s.split)
     system "./closure"
   end
 end

Formula/liboil.rb

@@ -33,7 +33,7 @@ class Liboil < Formula
         return 0;
       }
     EOS
-    flags = `#{HOMEBREW_PREFIX}/bin/pkg-config --cflags --libs liboil-0.3`.split + ENV.cflags.split
+    flags = `#{HOMEBREW_PREFIX}/bin/pkg-config --cflags --libs liboil-0.3`.split + ENV.cflags.to_s.split
     system ENV.cc, "test.c", "-o", "test", *flags
     system "./test"
   end

Formula/sqlite.rb

@@ -51,7 +51,7 @@ class Sqlite < Formula
                      "-dynamiclib",
                      "extension-functions.c",
                      "-o", "libsqlitefunctions.dylib",
-                     *ENV.cflags.split
+                     *ENV.cflags.to_s.split
       lib.install "libsqlitefunctions.dylib"
     end
     doc.install resource('docs') if build.with? "docs"