configure: fix basic syntax errors in the shell script programming language

There is no such thing as `+=` unless you happen to be using the Bash
programming language, i.e. your script shebang is /bin/bash.

However, configure scripts are run via /bin/sh instead, which may or may
not be be bash, usually depending on whether the system in question
preferred to have one less package installed (in which case it is bash)
vs. have a faster /bin/sh installed (in which case it is probably the
dash shell).

Author: eli-schwartz

configure.ac

@@ -66,7 +66,7 @@ ZMQ_ORIG_CXXFLAGS="${CXXFLAGS:-none}"
 
 # Checks for programs.
 AC_PROG_CC
-AX_CHECK_COMPILE_FLAG([-std=c11], [CFLAGS+=" -std=c11 -D_DEFAULT_SOURCE"], [AC_PROG_CC_C99])
+AX_CHECK_COMPILE_FLAG([-std=c11], [CFLAGS="${CFLAGS} -std=c11 -D_DEFAULT_SOURCE"], [AC_PROG_CC_C99])
 AC_PROG_CXX
 AX_CODE_COVERAGE
 AM_PROG_CC_C_O
@@ -87,8 +87,8 @@ AC_ARG_ENABLE([force-CXX98-compat],
     [AS_HELP_STRING([--enable-force-CXX98-compat], [force C++98 build [default=disabled]])])
 if test "x$enable_force_CXX98_compat" = "xyes"; then
     AC_LANG_PUSH([C++])
-    AX_CHECK_COMPILE_FLAG([-std=gnu++98], [CXXFLAGS+=" -std=gnu++98"], [])
-    AX_CHECK_COMPILE_FLAG([-Wc++98-compat -Wc++98-compat-pedantic], [CXXFLAGS+=" -Wc++98-compat"], [])
+    AX_CHECK_COMPILE_FLAG([-std=gnu++98], [CXXFLAGS="${CXXFLAGS} -std=gnu++98"], [])
+    AX_CHECK_COMPILE_FLAG([-Wc++98-compat -Wc++98-compat-pedantic], [CXXFLAGS="${CXXFLAGS} -Wc++98-compat"], [])
     AC_LANG_POP([C++])
 else
     AX_CXX_COMPILE_STDCXX_11([ext], [optional])
@@ -799,7 +799,7 @@ AM_CONDITIONAL(ON_GNU, test "x$libzmq_on_gnu" = "xyes")
 # Check for __atomic_Xxx compiler intrinsics
 AC_LANG_PUSH([C++])
 AX_CHECK_COMPILE_FLAG([-Watomic-alignment],
-    [CXXFLAGS+=" -Wno-atomic-alignment"],
+    [CXXFLAGS="${CXXFLAGS} -Wno-atomic-alignment"],
     [],
     [-Werror])
 LIBZMQ_CHECK_ATOMIC_INTRINSICS([
@@ -1137,12 +1137,12 @@ AM_CONDITIONAL(FUZZING_ENGINE_LIB, test "x$FUZZING_ENGINE_LIB" != "x")
 # clang 6 has a warning that does not make sense on multi-platform code
 AC_LANG_PUSH([C])
 AX_CHECK_COMPILE_FLAG([-Wtautological-constant-compare],
-    [CFLAGS+=" -Wno-tautological-constant-compare"],
+    [CFLAGS="${CFLAGS} -Wno-tautological-constant-compare"],
     [],
     [-Werror])
 AC_LANG_POP([C])
 AX_CHECK_COMPILE_FLAG([-Wtautological-constant-compare],
-    [CXXFLAGS+=" -Wno-tautological-constant-compare"],
+    [CXXFLAGS="${CXXFLAGS} -Wno-tautological-constant-compare"],
     [],
     [-Werror])
 
@@ -1151,12 +1151,12 @@ AX_CHECK_COMPILE_FLAG([-Wtautological-constant-compare],
 # E: lto-no-text-in-archive
 AC_LANG_PUSH([C])
 AX_CHECK_COMPILE_FLAG([-ffat-lto-objects],
-    [CFLAGS+=" -ffat-lto-objects"],
+    [CFLAGS="${CFLAGS} -ffat-lto-objects"],
     [],
     [-Werror])
 AC_LANG_POP([C])
 AX_CHECK_COMPILE_FLAG([-ffat-lto-objects],
-    [CXXFLAGS+=" -ffat-lto-objects"],
+    [CXXFLAGS="${CXXFLAGS} -ffat-lto-objects"],
     [],
     [-Werror])