build: fix setting CFLAGS on dash shell (Alpine Linux)

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>
1 file changed