Auto merge of #20894 - tigercosmos:rrr, r=avadacatavra

readme add llvm clang

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update readme, since recent commit required the dependencies.
part of #20888

---
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- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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README.md

@@ -83,7 +83,7 @@ sudo apt install git curl autoconf libx11-dev \
     gperf g++ build-essential cmake virtualenv python-pip \
     libssl1.0-dev libbz2-dev libosmesa6-dev libxmu6 libxmu-dev \
     libglu1-mesa-dev libgles2-mesa-dev libegl1-mesa-dev libdbus-1-dev \
-    libharfbuzz-dev ccache
+    libharfbuzz-dev ccache clang
 ```
 
 If you using a version prior to **Ubuntu 17.04** or **Debian Sid**, replace `libssl1.0-dev` with `libssl-dev`.
@@ -101,7 +101,7 @@ sudo dnf install curl libtool gcc-c++ libXi-devel \
     freetype-devel mesa-libGL-devel mesa-libEGL-devel glib2-devel libX11-devel libXrandr-devel gperf \
     fontconfig-devel cabextract ttmkfdir python python-virtualenv python-pip expat-devel \
     rpm-build openssl-devel cmake bzip2-devel libXcursor-devel libXmu-devel mesa-libOSMesa-devel \
-    dbus-devel ncurses-devel harfbuzz-devel ccache mesa-libGLU-devel
+    dbus-devel ncurses-devel harfbuzz-devel ccache mesa-libGLU-devel clang clang-libs
 ```
 #### On CentOS
 
@@ -110,20 +110,33 @@ sudo yum install curl libtool gcc-c++ libXi-devel \
     freetype-devel mesa-libGL-devel mesa-libEGL-devel glib2-devel libX11-devel libXrandr-devel gperf \
     fontconfig-devel cabextract ttmkfdir python python-virtualenv python-pip expat-devel \
     rpm-build openssl-devel cmake3 bzip2-devel libXcursor-devel libXmu-devel mesa-libOSMesa-devel \
-    dbus-devel ncurses-devel python34 harfbuzz-devel ccache
+    dbus-devel ncurses-devel python34 harfbuzz-devel ccache clang clang-libs llvm-toolset-7
+```
+
+Build inside `llvm-toolset` and `devtoolset`:
+
+```sh
+scl enable devtoolset-7 llvm-toolset-7 bash
+```
+
+with the following environmental variables set:
+
+```sh
+export CMAKE=cmake3
+export LIBCLANG_PATH=/opt/rh/llvm-toolset-7/root/usr/lib64
 ```
 #### On openSUSE Linux
 ``` sh
 sudo zypper install libX11-devel libexpat-devel libbz2-devel Mesa-libEGL-devel Mesa-libGL-devel cabextract cmake \
     dbus-1-devel fontconfig-devel freetype-devel gcc-c++ git glib2-devel gperf \
     harfbuzz-devel libOSMesa-devel libXcursor-devel libXi-devel libXmu-devel libXrandr-devel libopenssl-devel \
-    python-pip python-virtualenv rpm-build glu-devel ccache
+    python-pip python-virtualenv rpm-build glu-devel ccache llvm-clang libclang 
 ```
 #### On Arch Linux
 
 ``` sh
 sudo pacman -S --needed base-devel git python2 python2-virtualenv python2-pip mesa cmake bzip2 libxmu glu \
-    pkg-config ttf-fira-sans harfbuzz ccache
+    pkg-config ttf-fira-sans harfbuzz ccache clang 
 ```
 #### On Gentoo Linux
 
@@ -131,8 +144,14 @@ sudo pacman -S --needed base-devel git python2 python2-virtualenv python2-pip me
 sudo emerge net-misc/curl \
     media-libs/freetype media-libs/mesa dev-util/gperf \
     dev-python/virtualenv dev-python/pip dev-libs/openssl \
-    x11-libs/libXmu media-libs/glu x11-base/xorg-server \
-    media-libs/harfbuzz dev-util/ccache
+    media-libs/harfbuzz dev-util/ccache \
+    x11-libs/libXmu media-libs/glu x11-base/xorg-server sys-devel/clang
+```
+
+with the following environment variable set:
+
+```sh
+export LIBCLANG_PATH="/usr/lib64/llvm/*/lib64"
 ```
 #### On Windows (MSVC)