Avoid setting redundant CFLAGS to compile commands

[webmaster128]

In the 5 locations where $(CURL_CC) is used, $(CURL_CFLAGS) is always appended, so before this, all arguments in CURL_CFLAGS have been added twice.

Confirm the following snipped from the build log (indent added):

Copying libs...
    cl.exe /O2 /DNDEBUG /MD /DCURL_STATICLIB↩
        /I../lib /I../include /nologo /W3 /EHsc /DWIN32 /FD /c↩
        /I../lib /I../include /nologo /W3 /EHsc /DWIN32 /FD /c↩
        /Zm200 /Fo"..\builds\libcurl-vc14-x86-release-static-ipv6-sspi-winssl-obj-curl\tool_hugehelp.obj"↩
        ..\src\tool_hugehelp.c
tool_hugehelp.c
    cl.exe /O2 /DNDEBUG /MD /DCURL_STATICLIB↩
        /I../lib /I../include /nologo /W3 /EHsc /DWIN32 /FD /c↩
        /I../lib /I../include /nologo /W3 /EHsc /DWIN32 /FD /c↩
        /Fo"..\builds\libcurl-vc14-x86-release-static-ipv6-sspi-winssl-obj-curl\nonblock.obj"↩
        ../lib/nonblock.c
nonblock.c
    cl.exe /O2 /DNDEBUG /MD /DCURL_STATICLIB↩
        /I../lib /I../include /nologo /W3 /EHsc /DWIN32 /FD /c↩
        /I../lib /I../include /nologo /W3 /EHsc /DWIN32 /FD /c↩
        /Fo"..\builds\libcurl-vc14-x86-release-static-ipv6-sspi-winssl-obj-curl\rawstr.obj"↩
        ../lib/rawstr.c
rawstr.c
    cl.exe /O2 /DNDEBUG /MD /DCURL_STATICLIB↩
        /I../lib /I../include /nologo /W3 /EHsc /DWIN32 /FD /c↩
        /I../lib /I../include /nologo /W3 /EHsc /DWIN32 /FD /c↩
        /Fo"..\builds\libcurl-vc14-x86-release-static-ipv6-sspi-winssl-obj-curl\strtoofft.obj"↩
        ../lib/strtoofft.c
strtoofft.c
    cl.exe /O2 /DNDEBUG /MD /DCURL_STATICLIB↩
        /I../lib /I../include /nologo /W3 /EHsc /DWIN32 /FD /c↩
        /I../lib /I../include /nologo /W3 /EHsc /DWIN32 /FD /c↩
        /Fo"..\builds\libcurl-vc14-x86-release-static-ipv6-sspi-winssl-obj-curl\warnless.obj"↩
        ../lib/warnless.c
warnless.c

[mention-bot]

@webmaster128, thanks for your PR! By analyzing the annotation information on this pull request, we identified @yangtse, @mback2k and @BurningEnlightenment to be potential reviewers

[bagder]

thanks!