docs: Update build instructions to skip SDL2

The build system will select SDL2 by default, but it is currently
not supported by bflibrary. So, needs to be disabled while building.

README.md

@@ -191,7 +191,7 @@ Next, proceed with the build steps; we will do that in a separate folder.
 
 ```
 mkdir -p release; cd release
-PKG_CONFIG_PATH="/usr/lib/i386-linux-gnu/pkgconfig" CFLAGS="-m32" CXXFLAGS="-m32" LDFLAGS="-m32" ../configure
+PKG_CONFIG_PATH="/usr/lib/i386-linux-gnu/pkgconfig" CFLAGS="-m32" CXXFLAGS="-m32" LDFLAGS="-m32" ../configure --without-sdl2
 make V=1
 ```
 
@@ -204,7 +204,7 @@ In case you also want a debug build:
 
 ```
 mkdir -p debug; cd debug
-PKG_CONFIG_PATH="/usr/lib/i386-linux-gnu/pkgconfig" CPPFLAGS="-DDEBUG -D__DEBUG" CFLAGS="-m32 -g -O0 -Wall" CXXFLAGS="-m32 -g -O0 -Wall" LDFLAGS="-m32 -g -O0 -Wall" ../configure
+PKG_CONFIG_PATH="/usr/lib/i386-linux-gnu/pkgconfig" CPPFLAGS="-DDEBUG -D__DEBUG" CFLAGS="-m32 -g -O0 -Wall" CXXFLAGS="-m32 -g -O0 -Wall" LDFLAGS="-m32 -g -O0 -Wall" ../configure --without-sdl2
 make V=1
 ```
 
@@ -281,7 +281,7 @@ the default mingw64 ones:
 
 ```
 mkdir -p release; cd release
-PATH="/mingw32/bin:$PATH" PKG_CONFIG_PATH="/mingw32/lib/pkgconfig" CFLAGS="-m32" CXXFLAGS="-m32" LDFLAGS="-m32" ../configure --prefix=/mingw32
+PATH="/mingw32/bin:$PATH" PKG_CONFIG_PATH="/mingw32/lib/pkgconfig" CFLAGS="-m32" CXXFLAGS="-m32" LDFLAGS="-m32" ../configure --prefix=/mingw32 --without-sdl2
 PATH="/mingw32/bin:$PATH" make V=1
 ```