cflags for vendor variants

cflags that are only applied when building the vendor variant of a lib
is now supported. This will be particularily useful when existing macro
should be turned off for the vendor variant:

cflags: ["-DENABLE_FEATURE_X"],

target: {
    vendor: {
        cflags: ["-UENABLE_FEATURE_X"],
    },
},

Without this, the library author has to undef the macro inside the
source code using __ANDROID_VNDK__, which is not desired especially for
external projects.

Bug: 64514237
Test: BOARD_VNDK_VERSION=current m -j libsqlite libsqlite.vendor
Merged-In: Ib484b5b12e14ab776276a30ed2763dffadc9d642
Change-Id: Ib484b5b12e14ab776276a30ed2763dffadc9d642
(cherry picked from commit 1e7495d973)

cc/compiler.go

@@ -134,6 +134,10 @@ type BaseCompilerProperties struct {
 			// list of source files that should not be used to
 			// build the vendor variant of the C/C++ module.
 			Exclude_srcs []string
+
+			// List of additional cflags that should be used to build the vendor
+			// variant of the C/C++ module.
+			Cflags []string
 		}
 	}
 
@@ -396,6 +400,10 @@ func (compiler *baseCompiler) compilerFlags(ctx ModuleContext, flags Flags) Flag
 		flags.CppFlags = append([]string{"-std=" + cppStd}, flags.CppFlags...)
 	}
 
+	if ctx.vndk() {
+		flags.CFlags = append(flags.CFlags, esc(compiler.Properties.Target.Vendor.Cflags)...)
+	}
+
 	// We can enforce some rules more strictly in the code we own. strict
 	// indicates if this is code that we can be stricter with. If we have
 	// rules that we want to apply to *our* code (but maybe can't for