Fix LLVM related headers to compile standalone (to fix cpluspluscheck).

Previously llvmjit.h #error'ed when USE_LLVM was not defined, to
prevent it from being included from code not having #ifdef USE_LLVM
guards - but that's not actually that useful after, during the
development of JIT support, LLVM related code was moved into a
separately compiled .so.  Having that #error means cpluspluscheck
doesn't work when llvm support isn't enabled, which isn't great.

Similarly add USE_LLVM guards to llvmjit_emit.h, and additionally make
sure it compiles standalone.

Per complaint from Tom Lane.

Author: Andres Freund
Discussion: https://postgr.es/m/19808.1548692361@sss.pgh.pa.us
Backpatch: 11, where JIT support was added

src/include/jit/llvmjit.h

@@ -11,9 +11,11 @@
 #ifndef LLVMJIT_H
 #define LLVMJIT_H
 
-#ifndef USE_LLVM
-#error "llvmjit.h should only be included by code dealing with llvm"
-#endif
+/*
+ * To avoid breaking cpluspluscheck, allow including the file even when LLVM
+ * is not available.
+ */
+#ifdef USE_LLVM
 
 #include <llvm-c/Types.h>
 
@@ -140,4 +142,5 @@ extern char *LLVMGetHostCPUFeatures(void);
 } /* extern "C" */
 #endif
 
+#endif							/* USE_LLVM */
 #endif							/* LLVMJIT_H */

src/include/jit/llvmjit_emit.h

@@ -9,9 +9,17 @@
 #ifndef LLVMJIT_EMIT_H
 #define LLVMJIT_EMIT_H
 
+/*
+ * To avoid breaking cpluspluscheck, allow including the file even when LLVM
+ * is not available.
+ */
+#ifdef USE_LLVM
 
 #include <llvm-c/Core.h>
 
+#include "fmgr.h"
+#include "jit/llvmjit.h"
+
 
 /*
  * Emit a non-LLVM pointer as an LLVM constant.
@@ -263,4 +271,5 @@ l_funcvalue(LLVMBuilderRef b, LLVMValueRef v_fcinfo, size_t argno)
 	return LLVMBuildLoad(b, l_funcvaluep(b, v_fcinfo, argno), "");
 }
 
+#endif							/* USE_LLVM */
 #endif