configure.ac: set LLVM_{C, CXX, LD}FLAGS only as needed

Earlier refactoring commits started setting the above regardless if LLVM
is used or not. Move them to the respective section to restore the
original functionality.

Since we require the preprocessor flags (includes in particular) for the
header version parsing keep those as-is. They are not used outside of
configure.ac thus should not cause any side-effects.

As-is adding the C/CXXFLAGS can lead to build issues on when
cross-compiling.

Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: Tomasz Figa <tfiga@chromium.org>
Cc: "17.0" <mesa-stable@lists.freedesktop.org>
Reported-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Tobias Droste <tdroste@gmx.de>
(cherry picked from commit 4d8bb9cf8c8e8bd3185461760ca89e6021c0b8e0)

Conflicts:
	configure.ac
diff --git a/configure.ac b/configure.ac
index 3906ac7..4703b6a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -969,17 +969,14 @@
 llvm_set_environment_variables() {
     if test "x$LLVM_CONFIG" != xno; then
         LLVM_VERSION=`$LLVM_CONFIG --version | egrep -o '^[[0-9.]]+'`
-        LLVM_LDFLAGS=`$LLVM_CONFIG --ldflags`
         LLVM_BINDIR=`$LLVM_CONFIG --bindir`
         LLVM_CPPFLAGS=`strip_unwanted_llvm_flags "$LLVM_CONFIG --cppflags"`
-        LLVM_CFLAGS=$LLVM_CPPFLAGS   # CPPFLAGS seem to be sufficient
-        LLVM_CXXFLAGS=`strip_unwanted_llvm_flags "$LLVM_CONFIG --cxxflags"`
         LLVM_INCLUDEDIR=`$LLVM_CONFIG --includedir`
         LLVM_LIBDIR=`$LLVM_CONFIG --libdir`
 
-        # We need to respect LLVM_CFLAGS when compiling LLVM headers.
+        # We need to respect LLVM_CPPFLAGS when compiling LLVM headers.
         save_CFLAGS="$CFLAGS"
-        CFLAGS="$CFLAGS $LLVM_CFLAGS"
+        CFLAGS="$CFLAGS $LLVM_CPPFLAGS"
 
         AC_COMPUTE_INT([LLVM_VERSION_MAJOR], [LLVM_VERSION_MAJOR],
             [#include "${LLVM_INCLUDEDIR}/llvm/Config/llvm-config.h"])
@@ -2411,6 +2408,10 @@
 if test "x$enable_gallium_llvm" = xyes; then
     DEFINES="${DEFINES} -DHAVE_LLVM=0x0$LLVM_VERSION_INT -DMESA_LLVM_VERSION_PATCH=$LLVM_VERSION_PATCH"
 
+    LLVM_LDFLAGS=`$LLVM_CONFIG --ldflags`
+    LLVM_CFLAGS=$LLVM_CPPFLAGS   # CPPFLAGS seem to be sufficient
+    LLVM_CXXFLAGS=`strip_unwanted_llvm_flags "$LLVM_CONFIG --cxxflags"`
+
     dnl Set LLVM_LIBS - This is done after the driver configuration so
     dnl that drivers can add additional components to LLVM_COMPONENTS.
     dnl Previously, gallium drivers were updating LLVM_LIBS directly