configure.ac: Drop LLVM compiler flags more radically

Drop all -m*, -W*, -O*, -g* and -f* flags, with the exception of
-fno-rtti, which must be used if it's part of the llvm-config --cxxflags
output. We don't want LLVM to dictate the flags we use, and it can even
cause build failures, e.g. if LLVM and Mesa are built with different
compilers.

While we're at it, eat any whitespace preceding dropped flags as well.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit 0f53404565b9ef9da9d7022b5732463acd496742)
Nominated-by: Marek Olšák <marek.olsak@amd.com>
Bugzilla: https://bugs.freedesktop.org/100028
diff --git a/configure.ac b/configure.ac
index 399166a..bdcf14d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -943,27 +943,18 @@
 # $1 is the llvm-config command with arguments.
 strip_unwanted_llvm_flags() {
     # Use \> (marks the end of the word)
-    echo `$1` | sed \
-        -e 's/-march=\S*//g' \
-        -e 's/-mtune=\S*//g' \
-        -e 's/-mcpu=\S*//g' \
-        -e 's/-DNDEBUG\>//g' \
-        -e 's/-D_GNU_SOURCE\>//g' \
-        -e 's/-pedantic\>//g' \
-        -e 's/-Wcovered-switch-default\>//g' \
-        -e 's/-O.\>//g' \
-        -e 's/-g\>//g' \
-        -e 's/-Wall\>//g' \
-        -e 's/-Wcast-qual\>//g' \
-        -e 's/-Woverloaded-virtual\>//g' \
-        -e 's/-fcolor-diagnostics\>//g' \
-        -e 's/-fdata-sections\>//g' \
-        -e 's/-ffunction-sections\>//g' \
-        -e 's/-fno-exceptions\>//g' \
-        -e 's/-fomit-frame-pointer\>//g' \
-        -e 's/-fvisibility-inlines-hidden\>//g' \
-        -e 's/-fPIC\>//g' \
-        -e 's/-fstack-protector-strong\>//g'
+    echo " `$1`" | sed \
+        -e 's/\s\+-m\S*//g' \
+        -e 's/\s\+-DNDEBUG\>//g' \
+        -e 's/\s\+-D_GNU_SOURCE\>//g' \
+        -e 's/\s\+-pedantic\>//g' \
+        -e 's/\s\+-W\S*//g' \
+        -e 's/\s\+-O\S*//g' \
+        -e 's/\s\+-g\S*//g' \
+        -e 's/-fno-rtti\>/-Fno-rtti/g' \
+        -e 's/\s\+-f\S*//g' \
+        -e 's/-Fno-rtti\>/-fno-rtti/g' \
+        -e 's/^ //'
 }
 
 llvm_set_environment_variables() {