Syntax like "CFLAGS+=" does not work on simple shells, e.g. dash from
BusyBox.  This seems to fail to properly configure CFLAGS on Alpine
Linux:

    checking for pkg-config... /usr/bin/pkg-config
    checking pkg-config is at least version 0.9.0... yes
    ./configure: line 3441: CFLAGS+= -Werror -Wextra: not found
    ./configure: line 3442: CFLAGS+= -Wno-unused-parameter: not found
    ./configure: line 3443: CFLAGS+= -Wno-missing-field-initializers: not found
    ./configure: line 3444: CFLAGS+= -Wdeclaration-after-statement: not found
    ./configure: line 3445: CFLAGS+= -Wmissing-declarations: not found
    ./configure: line 3446: CFLAGS+= -Wredundant-decls: not found
    ./configure: line 3447: CFLAGS+= -Wcast-align: not found
    ./configure: line 3448: CFLAGS+= -DG_DISABLE_DEPRECATED: not found
    checking for gcc... gcc

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
---
 acinclude.m4 | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/acinclude.m4 b/acinclude.m4
index fbc7660dfb67..027ed69977db 100644
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -15,13 +15,14 @@ AC_DEFUN([NEARD_COMPILER_FLAGS], [
 		CFLAGS="-Wall -O2 -D_FORTIFY_SOURCE=2"
 	fi
 	if (test "$USE_MAINTAINER_MODE" = "yes"); then
-		CFLAGS+=" -Werror -Wextra"
-		CFLAGS+=" -Wno-unused-parameter"
-		CFLAGS+=" -Wno-missing-field-initializers"
-		CFLAGS+=" -Wdeclaration-after-statement"
-		CFLAGS+=" -Wmissing-declarations"
-		CFLAGS+=" -Wredundant-decls"
-		CFLAGS+=" -Wcast-align"
-		CFLAGS+=" -DG_DISABLE_DEPRECATED"
+		CFLAGS="$CFLAGS -Werror -Wextra"
+		CFLAGS="$CFLAGS -Werror -Wextra"
+		CFLAGS="$CFLAGS -Wno-unused-parameter"
+		CFLAGS="$CFLAGS -Wno-missing-field-initializers"
+		CFLAGS="$CFLAGS -Wdeclaration-after-statement"
+		CFLAGS="$CFLAGS -Wmissing-declarations"
+		CFLAGS="$CFLAGS -Wredundant-decls"
+		CFLAGS="$CFLAGS -Wcast-align"
+		CFLAGS="$CFLAGS -DG_DISABLE_DEPRECATED"
 	fi
 ])
-- 
2.27.0