Cleanup Bash scripts.

This makes them pass shellcheck. Except for a few unused
export warnings.

I also wonder if the majority of these files should be
in the root of the project, or should be moved to "build-scripts"
(or should still even exist).

a few minor changes

one more minor changes

build-debug-g3-sdl2

@@ -1,9 +1,10 @@
 #!/usr/bin/env bash
 
 # allow 32-bit on 64-bit (x86) builds
-if [ "$1" == "32" ]; then
-    export CC=`which gcc`" -m32"
-    export CXX=`which g++`" -m32"
+if [ "${1}" == "32" ]; then
+    CC="$(which gcc) -m32"
+    CXX="$(which g++) -m32"
+    export CC CXX
     shift
 fi
 
@@ -17,16 +18,17 @@ chmod +x vs/sdl/build-scripts/strip_fPIC.sh
 
 chmod +x configure
 
-export CFLAGS="$CFLAGS -g3 -O0 -fno-inline -fno-omit-frame-pointer"
-export CXXFLAGS="$CXXFLAGS -g3 -O0 -fno-inline -fno-omit-frame-pointer"
+CFLAGS="${CFLAGS} -g3 -O0 -fno-inline -fno-omit-frame-pointer"
+CXXFLAGS="${CXXFLAGS} -g3 -O0 -fno-inline -fno-omit-frame-pointer"
+export CFLAGS CXXFLAGS
 
 # prefer to compile against our own copy of SDL 2.x IF the system does not provide one
-if [[ "$no_host_sdl2" ]]; then
+if [[ "${no_host_sdl2}" ]]; then
     x= # doesn't work well cross-compiling
 else
-    x=`which sdl2-config`
+    x=$(which sdl2-config)
 fi
-if test -z "$x" ; then
+if test -z "${x}" ; then
     echo Compiling our internal SDL 2.x
     (cd vs/sdl2 && ./build-dosbox.sh) || exit 1
 fi
@@ -36,6 +38,6 @@ echo Compiling our internal SDLnet 1.x
 (cd vs/sdlnet && ./build-dosbox.sh) || exit 1
 
 # NTS: --disable-dynamic-core is needed. the dynamic core doesn't work properly with the CFLAGS given above
-./configure --enable-debug=heavy --prefix=/usr --disable-dynamic-core --enable-sdl2 "$@" || exit 1
+./configure --enable-debug=heavy --prefix=/usr --disable-dynamic-core --enable-sdl2 "${@}" || exit 1
 make -j3 || exit 1