configure: avoid non-POSIX: test ... -a/-o ...

Shells do support those, and typically simple forms do work, but
these were removed in POSIX 2024 because they are notoriously hard
to parse correctly, and shells don't always agree on the result.

Instead, use standard forms which all shells support identically.

Extreme high level overview of quotes in POSIX shell:
- WORDs without $... or glob don't need quotes (echo, =, no, x86, ...).
- a=  b=foo  c=$...  and  case $d in ...  all don't need quotes.
- IFS/glob affect command and arguments (not assignments or case..in):
  - IFS only splits direct result of $..., quotes will prevent that.
  - Glob then splits/matches *?[]      and quotes will prevent that.

POSIX "test" (see Application Usage), and shell language (quotes):
  https://pubs.opengroup.org/onlinepubs/9799919799/utilities/test.html
  https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html

My own quick guide:
  https://gist.github.com/avih/6752ad1e20b334b56fef120cd09c766e

configure

@@ -210,7 +210,7 @@ default() # set variable unless already set
 {
     local v
     eval v=\"\$$1\"
-    test -z "$v" -a -n "$2" && eval $1=\"$2\"
+    test -z "$v" && test -n "$2" && eval $1=\"$2\"
 }
 
 default_conf() # add to confvars unless already present
@@ -219,7 +219,7 @@ default_conf() # add to confvars unless already present
     test "${confvars%$v*}" = "${confvars}" && confvars="$confvars $1"
 }
 
-if test -z "$source_path" -o "$source_path" = "." ; then
+if test -z "${source_path#.}" ; then
   source_path=$(pwd)
   source_path_used="no"
 else
@@ -254,9 +254,10 @@ fi
 default cpu "$cpu_sys"
 
 # check for crpss build
-if test "$cpu" != "$cpu_sys" \
-  -o "$targetos" != "$buildos" \
-  -o -n "$cross_prefix" ; then
+if test "$cpu" != "$cpu_sys" ||
+   test "$targetos" != "$buildos" ||
+   test -n "$cross_prefix"
+then
   build_cross="yes"
   cc="${cross_prefix}${cc}"
   ar="${cross_prefix}${ar}"
@@ -337,7 +338,7 @@ case $targetos in
       fi
     fi
     # on OSX M1 with --cpu=x86_64, build a tcc to run under rosetta entirely
-    if test "$cpu" = "x86_64" -a "$cpu_sys" = "arm64"; then
+    if test "$cpu" = x86_64 && test "$cpu_sys" = arm64; then
         CFLAGS="$CFLAGS -arch $cpu"
         LDFLAGS="$LDFLAGS -arch $cpu"
     fi
@@ -422,17 +423,17 @@ if test -z "$build_cross"; then
     _triplet="$($CONFTEST triplet)"
   fi
   if test "$mingw32" = "no" ; then
-      if test -z "$triplet" -a -n "$_triplet"; then
+      if test -z "$triplet" && test -n "$_triplet"; then
         if test -f "/usr/lib/$_triplet/crti.o"; then
           triplet="$_triplet"
         fi
       fi
       if test -z "$triplet"; then
-        if test $cpu = "x86_64" -o $cpu = "arm64" -o $cpu = "riscv64" ; then
+        case $cpu in x86_64|arm64|riscv64)
           if test -f "/usr/lib64/crti.o" ; then
             tcc_lddir="lib64"
           fi
-        fi
+        esac
       fi
       if test "$suggest" = "yes"; then
         if test -f "/lib/ld-uClibc.so.0" ; then