Remove bash-isms from configure script

--- configure.ac.orig	2020-06-15 12:45:49 UTC
+++ configure.ac
@@ -154,7 +154,7 @@ AS_COMPILER_FLAGS(GLOBAL_CFLAGS, "-Wall -Wextra -Wredu
 GLOBAL_LDFLAGS="$PTHREAD_LIBS"
 
 if test "x$ac_cv_fmin_builtin" != "xyes"; then
-	GLOBAL_LDFLAGS+=" -lm"
+    GLOBAL_LDFLAGS="$GLOBAL_CFLAGS -lm"
 fi
 
 AC_ARG_ENABLE(debug,
@@ -169,7 +169,7 @@ esac],
 
 if (test "x$debug" = "xyes"); then
        AC_DEFINE(DEBUG, 1, [Define if debug message output code should be built.])
-       GLOBAL_CFLAGS+=" -g"
+       GLOBAL_CFLAGS="$GLOBAL_CFLAGS -g"
 fi
 
 AC_SUBST(GLOBAL_CFLAGS)
@@ -201,23 +201,23 @@ fi
 
 if test "x$build_sanitizers" = "xyes"; then
     AS_COMPILER_FLAG([-fsanitize=address], [
-        SANITIZER_FLAGS+=" -fsanitize=address"
+        SANITIZER_FLAGS="$SANITIZER_FLAGS -fsanitize=address"
         ASAN_AVAILABLE=yes
     ], [])
     if test "$ASAN_AVAILABLE" = "yes"; then
         AS_COMPILER_FLAG([-fsanitize=address -fsanitize-address-use-after-scope], [
-            SANITIZER_FLAGS+=" -fsanitize-address-use-after-scope"
+            SANITIZER_FLAGS="$SANITIZER_FLAGS -fsanitize-address-use-after-scope"
         ], [])
-        SANITIZERS+="ASAN "
+        SANITIZERS="$SANITIZERS ASAN"
     fi
 
     AS_COMPILER_FLAG([-fsanitize=undefined], [
-        SANITIZER_FLAGS+=" -fsanitize=undefined"
+        SANITIZER_FLAGS="$SANITIZER_FLAGS -fsanitize=undefined"
         UBSAN_AVAILABLE=yes
     ], [])
 
     if test "$UBSAN_AVAILABLE" = "yes"; then
-        SANITIZERS+="UBSAN "
+        SANITIZERS="$SANITIZERS UBSAN"
     fi
 
     if test -z "$SANITIZER_FLAGS"; then
@@ -230,13 +230,13 @@ if test "x$build_sanitizers" = "xyes"; then
             if test -z "$SAN_COV_FLAGS"; then
                 SAN_COV_FLAGS="$COV_CHECK"
             else
-                SAN_COV_FLAGS+=",$COV_CHECK"
+                SAN_COV_FLAGS="$SAN_COV_FLAGS,$COV_CHECK"
             fi
         ], [])
     done
     if test -n "$SAN_COV_FLAGS"; then
-        SANITIZER_FLAGS+=" -fsanitize-coverage=$SAN_COV_FLAGS"
-        SANITIZERS+="+coverage "
+        SANITIZER_FLAGS="$SANITIZER_FLAGS -fsanitize-coverage=$SAN_COV_FLAGS"
+        SANITIZERS="$SANITIZERS +coverage"
     else
         AC_MSG_WARN([No sanitizer coverage supported by compiler])
     fi
@@ -244,20 +244,20 @@ if test "x$build_sanitizers" = "xyes"; then
     CFLAGS="-O1"
 
     AS_COMPILER_FLAG([-fno-omit-frame-pointer], [
-        CFLAGS+=" -fno-omit-frame-pointer"
+        CFLAGS="$CFLAGS -fno-omit-frame-pointer"
     ], [])
 
     AS_COMPILER_FLAG([-gline-tables-only], [
-        CFLAGS+=" -gline-tables-only"
+        CFLAGS="$CFLAGS -gline-tables-only"
     ],
     [
-        CFLAGS+=" -g"
+        CFLAGS="$CFLAGS -g"
     ])
 
-    EXTRA_CONF+="  Enabled sanitizers ......: $SANITIZERS
+    EXTRA_CONF="$EXTRA_CONF  Enabled sanitizers ......:$SANITIZERS
 "
 
-    CFLAGS+=" $SANITIZER_FLAGS"
+    CFLAGS="$CFLAGS $SANITIZER_FLAGS"
     CXXFLAGS="$CFLAGS -std=c++11"
 fi
 
@@ -266,9 +266,9 @@ if test "x$build_fuzzers" = "xyes"; then
         AC_MSG_WARN([building fuzzers requires clang/clang++ (continuing anyway)])
     fi
 
-    CFLAGS+=" -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION"
+    CFLAGS="$CFLAGS -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION"
 
-    EXTRA_CONF+="  Build fuzzers ...........: yes
+    EXTRA_CONF="$EXTRA_CONF  Build fuzzers ...........: yes
 "
 fi